Zircon U-Pb and Hf isotopic constraints on the magmatic evolution of the Northern Luzon Arc

The complete volcanic sequences restored in the Coastal Range of Taiwan are key archives for better understanding the magmatic and tectonic evolution of the Northern Luzon Arc. This paper reports (1) new zircon U-Pb ages and Hf isotopic data of fourteen volcanic samples from different sequences of four major volcanoes in the Coastal Range, (2) Hf isotopic data of dated magmatic and detrital zircons from two offshore volcanic islands, Lutao and Lanyu. These data indicate that the arc magmatism in the Coastal Range started at ~15 Ma, most active at ~9 Ma, and ceased at ~4.2 Ma. Magmatic zircons from the arc rocks show a significant variation in Hf isotopic composition, with εHf(T) values varying from +24.9 to +4.8. As pointed out by our previous studies, old continental zircons that show Cathaysian-type ages and Hf isotope features are common in samples from the Yuemei, Chimei, and Lanyu volcanoes, supporting the notion for the influence of the existence of an accreted micro-continent or continental fragment plays a role in the petrogenesis. Such inherited zircons are not observed in the Chengkuang’ao and Tuluanshan volcanoes and uncommon in Lutao, implying the discontinuity or a limited extent of the accreted continental fragment. The εHf(T) values are high and positive from ~15 8 Ma (+25 to +15; ±5ε-unit variation), and became lower from ~6 to 4.2 Ma (+20 to +8; ±6ε units) and the lowest from ~1.3 Ma (+19 to +5; ±7ε units). Such a temporal variation in zircon Hf isotopic ratios can be also identified in whole-rock Hf and Nd isotopic compositions, which decrease from ~6 Ma when the Northern Luzon Arc may have started colliding with the Eurasian continental margin. Article history:


INTRodUCTIoN
The Northern Luzon Arc (NLA) consists of the Coastal Range (CR) of Taiwan and volcanic islands between Taiwan and Luzon Island, e.g., the Lutao and Lanyu Islands (Fig. 1a).The CR refers to the region that has collided with the Eurasian continental margin, and it consists of volcanoes with complete volcanic sequences that can be used to study the magmatic and tectonic evolution of the NLA.From north to south, the four major volcanoes in the CR were recognized as the Yuemei, Chimei, Chengkuang'ao, and Tuluanshan volcanic groups by Lai and Song (2013) (Fig. 1b).Previous studies have produced abundant dating results for the volcanic rocks in the CR; however, geologists need to use these data carefully because of two major questions.The first problem with many geochronological studies is the inaccuracy of the K/Ar results, due to the lowtemperature alteration experienced by these samples (Lo et al. 1994).Another problem is that the dating results from the same location are inconsistent with each other (Ho 1969;Juang and Bellon 1984;Richard et al. 1986;Yang et al. 1988Yang et al. , 1995;;Song 1990;Lo et al. 1994;Song and Lo 2002).This is because volcanic sequences were not established until Song and Lo (1988), and volcanic rocks from different sequences may yield various ages even in the same outcrop.Consequently, these previous ages are considered for reference only.Recently, Shao et al. (2015) used the results of Terr. Atmos. Ocean. Sci., Vol. 29, No. 2, 149-186, April 2018 zircon U-Pb dating to constrain the beginning of the main magmatism of the Chimei volcano to ~9 Ma; on the other hand, Lai et al. (2017) reported that the 40 Ar/ 39 Ar ages from the top sequences of each volcano in the CR represent the time of its latest eruption.
Zircon is a mineral that is physically and chemically robust under wide-ranging conditions and is widespread in volcanic rocks.The closure temperature of the U-Pb isotopic system in zircon is very high; therefore, it can still yield accurate age information following hydrothermal alteration.For these reasons, the zircon U-Pb dating method is suitable for the analysis of volcanic rocks in the CR.In this paper, we report the in situ analyses of zircon U-Pb ages and Lu-Hf isotopes for volcanic rocks from individual volcanic sequences of these four major volcanoes in the CR.These results reflect the occurrence of several significant and different volcanic processes in each volcano.Moreover, we combine the ages and Hf isotopic compositions of volcanic rocks from the Lutao and Lanyu Islands.The inherited zircons from the Yuemei, Chimei, and Lanyu volcanoes yield Cathaysian-type ages and Hf isotopic compositions.Based on the ages and isotopic compositions of these six volcanoes, the petrogenesis of the NLA can be determined.

GeoLoGICAL BACkGRoUNd
The NLA was formed when the South China Sea Plate subducted beneath the Philippine Sea Plate from approximately 35 -17 Ma (Taylor and Hayes 1983).The CR in-cludes four major volcanoes, named the Yuemei, Chimei, Chengkuang'ao, and Tuluanshan volcanic groups, which have already become a part of Taiwan, as well as two volcanic islands located near Taiwan, which are named Lutao and Lanyu Islands (Fig. 1).Previous studies of the magmatic rocks in the CR have focused on their mineralogy and petrology (Wang 1966;Yen 1967), volcanology (Song and Lo 2002;Lai and Song 2013), geochemistry (Chen 1975;Lan 1982;Chen et al. 1990;Song 1990;Yang 1992;Lai et al. 2008Lai et al. , 2017;;Lai and Song 2013), and geochronology (Ho 1969;Juang and Bellon 1984;Richard et al. 1986;Yang et al. 1988Yang et al. , 1995;;Song 1990;Lo et al. 1994;Song and Lo 2002;Shao et al. 2015;Lai et al. 2017).Among these studies, the establishment of volcanic sequences and geochronologic analyses of volcanic rocks are the two main methods that have been used to understand the relationships between geochemical variations and time.
Volcanic sequences in the CR were first established by Hsu (1956); they were then revised by later geologists several times.The sequences used in this paper were established by Song and Lo (1988).These volcanic sequences include the Chimei Igneous Complex (CM) in the lower part and the Tuluanshan Formation in the upper part (Ho 1969; Song and Lo 1988).The Chimei Igneous Complex is composed of basaltic to andesitic lava flows and some volcaniclastic rocks (Yen 1967; Song and Lo 1988;Chen 1997).During the period of CM sequence, the volcanic island erupted below the volatile fragmentation depth and formed lava flows that were several thousand meters thick.The previous dating of volcanic (a) (b)  -c.
rocks in this sequence yielded ages of 29.7 -9.2 Ma using the K/Ar method (Ho 1969;Richard et al. 1986) and 16.4 -8.3 Ma using zircon fission track dating (Yang et al. 1988(Yang et al. , 1995)).Recently, Shao et al. (2015) obtained the youngest grains of zircons yielding the mean 206 Pb/ 238 U date at ~9 Ma in the CM sequence from Chimei volcano, which represents the emplacement age of the magma.The Tuluanshan Formation include three subsequences; from the lower part to the upper part, these are named the Shihmen Volcanic Breccia (SM), Shihtiping Tuff (STP), and Kangkou Limestone (Yen 1967; Song and Lo 1988).The SM sequence is composed of mono-to heterolithologic volcanic breccias, some tuff breccias, and occasional lava flows.It was formed by a more violent eruption when a volcanic island grew above the volatile fragmentation depth (Song 1990).The STP sequence conformably overlies the SM sequence.It represents the top of these volcanic sequences and contains ignimbrite, white tuff, and peperite (Song and Lo 1988;Lai and Song 2013).Its impacted structure, plastic deformation, and welded structure indicate that the STP sequence was formed within a subaerial environment (Song 1990;Lai and Song 2013).The results of the 40 Ar/ 39 Ar dating of the ignimbrite in this sequence were reported by Lai et al. (2017), and include ages of ~7.2 Ma in Yuemei volcano, ~4.2 Ma in Chimei volcano, and ~6.2 Ma in Chengkuang'ao volcano.Other dating studies that used the K/Ar and 40 Ar/ 39 Ar methods yielded inconsistent or inaccurate ages due to sample weathering, low-temperature alteration, and the analysis of samples from unknown sequences (Lo et al. 1994;Lai et al. 2017).Abundant geochemical data in the NLA have been reported by previous studies (Chen 1975;Lan 1982;Jacques 1987;Vidal et al. 1989;Chen et al. 1990;Defant et al. 1990;Song 1990;Yang 1992;Yang et al. 1992;McDermott et al. 1993;Fourcade et al. 1994;Marini et al. 2005;Lai et al. 2008Lai et al. , 2017;;Shao et al. 2015).To summarize the magma genesis models from previous studies, magmas from the NLA underwent only source contamination (Chen et al. 1990;Defant et al. 1990;McDermott et al. 1993;Marini et al. 2005) or crustal contamination before they were erupted (Yang 1992;Fourcade et al. 1994;Shao et al. 2015;Lai et al. 2017).Recently, Shao et al. (2015) reported the magmatic ages of the bottom sequence in Chimei volcano and proposed that the inherited zircons originated from the old continental crust of the Cathaysia Block.Furthermore, Lai et al. (2017) reported that the effects of crustal contamination increased after 6 Ma, according to the whole-rock Sr-Nd isotopic data.

SAmPLeS
Fourteen volcanic samples were analyzed during this study (Table 1).These samples were collected from the CM, SM, and STP sequences from the four major volcanoes of the CR (Figs. 1b and 2).At Yuemei volcano, we selected one sample (no.LD) from the SM sequence and another one (no.130811-6) from the STP sequence at the Lindging section (Figs.1b and 2a).Four samples were selected from Chimei volcano, one of which was from the Shihmen section (no.SM-1), which belongs to the SM sequence; the others were selected from the STP sequence at the Shihtiping section (no.STP-1, 130812-3, and STPW).In order to obtain complete data from whole volcanic sequences in this volcano, we also combined these data with the dating analyses of the CM sequence at the Hsiukulunchi section (no.6,10,17,20,and 24) reported by Shao et al. (2015) (Figs. 1b and 2b).A total of seven samples were collected from Chengkuang'ao volcano (Figs.1b and 2c).Two samples were selected from lava flows in the lower part of the SM sequence at the Mawukuchi (no.TH-1) and Sanhsienchi (no.SSCL) sections.Four samples were selected from the Taiyuan section, two of which were from the SM sequence (no.SM-2 and TYB) and two of which were from the STP sequence (no.STP-2 and TYW).The remaining sample was collected from the STP sequence in the Tunghochiao section (no.THCW).Only one sample was collected from Tuluanshan volcano, which was collected from the STP sequence (no.CLCW) at the Chilichi section (Figs.1b and 2c).All fourteen samples were fresh, homogeneous and considered to be representative of the volcanic sequences to which they belonged.Zircons from all samples were analyzed for both their U-Pb ages and Hf isotope data, except for one sample from the SM sequence at the Lingding section (no.LD), which was not analyzed for its Hf isotopic data because its zircons were exhausted after being dated.

ANALyTICAL meTHodS
Zircons were separated from all volcanic samples using conventional techniques, combining magnetic separation and heavy liquid methods.They were then mounted in epoxy and polished to expose the interiors of crystals for analysis.Cathodoluminescence (CL) images were taken at the Institute of Earth Sciences, Academia Sinica, Taipei, in order to examine the internal structures of individual zircons to select suitable positions for U-Pb dating and Lu-Hf isotope analyses.
In situ zircon U-Pb isotope dating analyses were performed using an Agilent 7500s inductively coupled plasma mass spectrometer (ICP-MS) attached to a New Wave UP213 laser ablation system at the Department of Geosciences, National Taiwan University.The laser repetition rate was 4 Hz, and the spot diameter was ~30 to 40 μm.Calibrations were performed by analyzing the GJ-1 zircon standard (608.5 ± 0.4 Ma, Jackson et al. 2004).Two other well-known zircon standards, 91500 (1065.4± 0.6 Ma, Wiedenbeck et al. 1995) andMud Tank (732 ± 5 Ma, Black andGulson 1978), as well as one new standard, Plešovice (337.1 ± 0.4 Ma, Sláma et al. 2008), were used for data quality control.
The operating conditions and detailed analytical procedures of these LA-ICPMS analyses have been described by Chiu et al. (2009).U-Th-Pb isotopic ratios were calculated using the GLITTER 4.4 (GEMOC) software, and the relative standard deviations of the GJ-1 reference values were set at 2%.The presence of common lead was directly corrected using the common lead correction function proposed by Andersen (2002).The weighted mean U-Pb ages, concordia plots, probability curves, and histogram plots were constructed using Isoplot v. 3.70 (Ludwig 2009).Because modern-day 235 U comprises less than 1% of natural U, relatively little 207 Pb was produced during the Phanerozoic (cf.Ireland and Williams 2003).Consequently, 207 Pb/ 206 Pb ages are used only for inherited zircons that are older than 1000 Ma, and 206 Pb/ 238 U ages are used to indicate the crystallization ages of younger zircons.
In situ Lu-Hf isotopic analyses were performed using a Thermo Finnigan Neptune multicollector-ICPMS attached to a New Wave UP193FX laser ablation system at the Department of Geosciences, National Taiwan University.Lu-Hf isotopes were measured on the dated locations of individual zircons with a spot diameter of ~50 μm.Calibrations were performed using the Mud Tank zircon standard ( 176 Hf/ 177 Hf = 0.282498 ± 0.000035, Lee et al. unpublished data).Detailed descriptions of these analytical techniques can be found in Wu et al. (2006).To avoid the isobaric inter-ference between 176 Yb and 176 Hf, we discarded all analyses in which 176 Yb/ 177 Hf was > 0.2.

Zircon U-Pb Ages
The zircon U-Pb age results are listed in Table A1.The significant mean age results of concordia diagrams, histograms and zircon CL images of each sample are plotted in Figs. 3 -7.Only a few zircons from the Chengkuang'ao volcano were analyzed for both their core and rim ages, because the zircons in the samples collected from Yuemei and Chimei volcanoes are generally too small to analyze repeatedly (i.e., they have diameters of < 100 μm).Moreover, although the zircons from the Chengkuang'ao and Tuluanshan volcanoes are large (with diameters of ~150 -300 μm), no apparent age variations between their cores and rims were observed.Many inherited zircons were found in Yuemei and Chimei volcanoes, but few were found in Chengkuang'ao and Tuluanshan volcanoes.The histograms of all analyzed ages are shown in Fig. 8 with the order of volcanic sequences from top to bottom, i.e., Yuemei volcano: STP, SM (Figs.Wang et al. 2005Wang et al. , 2007;;Chen et al. 2008;Li et al. 2014).Ages from the CM sequences in the Chimei volcano were published by Shao et al. (2015).
volcano, which were published by Shao et al. (2015), the other results were collected from fourteen samples and comprise a total of 448 individual zircon analyses.

Sm Sequence
One volcanic breccia sample (no.LD, Fig. 3a) yielded only 12 zircon grains from the SM sequence.These zircons are irregular in shape, with lengths ranging from 60 -80 μm, and they display oscillatory bands and cloudy-zoned CL image patterns (Fig. A1a).These zircons have relatively high U concentrations (155 -2169 ppm), and their Th/U ratios (0.15 -1.54) are higher than those of metamorphic zircons (Rubatto and Gebauer 2000) (Table A1).All of these zircons are inherited zircons and do not have Cenozoic ages.Except for one grain (LD-11), which yields an old age of 2308 Ma, all other zircons yielded ages ranging from 236 -812 Ma (Fig. 8b).

Cm Sequence
The zircon U-Pb ages of volcanic rocks in this sequence was analyzed by Shao et al. (2015), and we use them in the discussion section.

Sm Sequence
Only one volcanic breccia (no.SM-1, Fig. 4b) from the SM sequence was analyzed for its zircon U-Pb ages.A total of 15 zircon grains exhibit rounded to subrounded shapes and range in size from 50 -80 μm.Many of these zircons show CL images with oscillatory bands and cloudy-zoned patterns (Fig. A1b).The U concentrations range from 196 -753 ppm and exhibit Th/U ratios (0.2 -1.89) that are characteristic of magmatic zircons (Table A1).Only one Miocene age was presented by grain SM1-03, which yielded a 206 Pb/ 238 U age of 7.9 ± 0.7 Ma. (Fig. 8d)
In no.STP-1, only 9 zircon grains were separated; they did not yield significant ages (Fig. 4b).Their U concentrations are heterogeneous and range from 44 -1519 ppm, and their Th/U ratios range from 0.11 -1.85, thus indicating that they are magmatic zircons (Table A1).

Lava Flows in the Sm Sequence
Zircon grains were separated from two lava flow samples (no.SSCL and TH-1, Figs.5a -d) from the SM sequence for U-Pb dating.These zircons are dominated by euhedral shapes and range in size from 100 -250 μm.Most zircons reveal unzoned patterns, oscillatory bands, and resorbed cores in CL images (Figs.5b and d).
Combining the dating results of these two samples reveals that few inherited zircons can be found.Except for six concentrated ages ranging from 2726 -2541 Ma, other inherited ages are variable.Moreover, an obvious age peak yields a mean age of 8.3 ± 0.3 Ma (n = 33, MSWD = 1.5) (Fig. 8h).

Volcanic Breccias in the Sm Sequence
Zircons grains were separated from two volcanic breccias (no.TYB and SM-2, Figs.5e -h) from the SM sequence for U-Pb age analyses.These zircons are characterized by euhedral, prismatic and rounded shapes with diameters ranging from 100 -300 μm (Figs.5f and h).The CL images of these zircons show unzoned patterns, oscillatory bands, cloudy-zoned patterns, and resorbed cores, with aspect ratios ranging from 1:1 to 4:1.
Combining these two samples reveals that almost no inherited zircons are present in this sequence.On the other hand, the Cenozoic ages form a concordant cluster, yielding a mean 206 Pb/ 238 U age of 7.3 ± 0.2 Ma (n = 55, MSWD = 1.5) (Fig. 8g).

STP Sequence
Zircon grains from three volcanic bombs (no.THCW, STP-2, and TYW, Figs.6a -f) from the STP sequence were analyzed for their U-Pb ages.Most of the zircons separated from these rock samples are euhedral and range in length from 100 -300 μm.Their CL images show unzoned patterns, oscillatory bands and cloudy-zoned patterns, with aspect ratios ranging from 1:1 to 3:1 (Figs.6b, d, and f).
Combining all the dates from these three samples reveals that a few inherited zircons can be found.Four of the inherited ages show uniform range from 225 -245 Ma.Moreover, an obvious age peak yields a mean age of 7.0 ± 0.2 Ma (n = 79, MSWD = 4.3) (Fig. 8f).
T DM C refers to the zircon Hf crustal model ages, which are based on a depleted mantle (DM) source and the assumption that the host magma of the zircons has the average continental 176 Lu/ 177 Hf ratio of 0.015 (Griffin et al. 2002).All individual volcanic sequences in these four major volcanoes record significantly variable ε Hf (T) values that exceed the external analytical errors of the method (~3-ε units), which implies that the protolith magma in the NLA has a heterogeneous isotopic composition.

Cm Sequence
The zircon Hf isotopic data of volcanic rocks in this sequence was analyzed by Shao et al. (2015), and we use them in the discussion section.

Sm Sequence
Only a few zircons from one sample (no.SM-1, n = 4) of the SM sequence were analyzed for their Hf isotopic compositions after being dated; they yielded ε Hf (T) values ranging from +10.1 to +3.5 (Fig. 9b and Table A2).

Lava Flows in the Sm Sequence
Two lava flow samples from Chengkuang'ao volcano were analyzed for their Hf isotopic compositions (Table A2).Twelve spots from sample no.SSCL yielded ε Hf (T) values ranging from +24.9 to -24.6; however, zircons recording Late Miocene ages, ranging from 10.0 -8.2 Ma, yielded a relative uniform and positive ε Hf (T) values (ranging from +24.9 to +16.4) (Fig. 9c).A total of 23 Hf isotopic analyses were obtained from another sample (no.TH-1) exhibiting zircon ages ranging from 8.6 -6.4 Ma; they yielded similar ε Hf (T) values (ranging from +23.4 to +16.7) as the former sample (Fig. 9c).

Volcanic Breccias in the Sm Sequence
Two volcanic breccia samples were analyzed for their Hf isotopic compositions (Fig. 9c and Table A2).A total of 24 Hf isotopic analyses were obtained from zircons in sample no.TYB (9.0 -7.0 Ma).Their ε Hf (T) values are uniform and range from +20.2 to +15.1.Another sample no.SM-2 (8.0 -6.7 Ma) were analyzed 20 Hf isotopic ratios.They also have uniform ε Hf (T) values ranging from +19.6 to +17.5.

STP Sequence
Zircons from three samples were analyzed for their Hf isotopic compositions on previous U-Pb age spots; all data are plotted and listed in Fig. 9c and Table A2.In sample no.TYW, 24 Hf isotopic analyses from zircons with ages ranging from 8.8 -6.9 Ma yield uniform ε Hf (T) values (ranging from +20.0 to +15.4).Sample no.STP-2 yields 18 Hf isotopic analyses, which also show similar zircon ages (11.0 -6.0 Ma) and uniform ε Hf (T) values (ranging from +22.0 to +15.5), except for one inherited zircon, which has an age of 430 Ma [ε Hf (T) value = +4.8].The other sample no.THCW yields 24 Hf isotopic analyses of zircons with ages ranging from 7.0 -5.3 Ma; their ε Hf (T) values range from +16.2 to +8.9.

STP Sequence
We only collected one sample from the STP sequence of Tuluanshan volcano, which yields a population mean age of 8.5 ± 0.2 Ma and uniform ε Hf (T) values ranging from +21.6 to +16.9 (Fig. 9c and Table A2).Shao et al. (2014) published the results of U-Pb dating of both volcanic rocks and sands at Lutao volcano.We analyzed the 176 Hf/ 177 Hf isotopic ratios of zircons based on these ages.The Hf isotopic ratios record variable ε Hf (T) values, ranging from +17.8 to -4.5 in volcanic rocks and from +18.1 to -10.2 in sands (Fig. 9d and Table A2).The volcanic rocks yield a Quaternary age peak of 1.31 ± 0.03 Ma (Table 2) and positive ε Hf (T) values ranging from +17.8 to +5.2.The sands yield an age peak of 1.23 ± 0.03 Ma and positive ε Hf (T) values ranging from +18.1 to +4.9.The inherited zircons yield a range of ages, ranging from ca. 100 -270 Ma, as well as ε Hf (T) values ranging from +6.2 to -10.2.They also yield an older range of crustal model ages, ranging from ca. 2700 -1870 Ma, with ε Hf (T) values ranging from +8.0 to -4.5.

Lanyu Volcano
Shao et al. ( 2014) also performed U-Pb age dating in Lanyu volcano.We analyzed the 176 Hf/ 177 Hf isotopic ratios of zircons based on the dated positions of these samples.For all zircons, the ε Hf (T) values of volcanic rocks range from +20.0 to -9.4 and those of sands range from +21.3 to +3.0 (Fig. 9d and Table A2).Zircons recording a mean Quaternary age of 2.61 ± 0.13 Ma (Table 2) yield uniform and positive ε Hf (T) values ranging from +12.8 to +12.3 in the volcanic rocks, and those exhibiting a mean age of 2.69 ± 0.11 Ma yield positive ε Hf (T) values ranging from +21.3 to +7.5 in the sands.In general, of all the analyzed data, inherited zircons showing ε Hf (T) values ranging from +6.1 to -9.4 yield a range of ages, ranging from ca. 160 -100 Ma; zircons recording ε Hf (T) values ranging from +17.9 to -5.5 yield an older range of crustal model ages, ranging from ca. 2600 -1920 Ma.

Zircon U-Pb Ages and Hf Isotopes in the Cm
Sequence of the Chimei Volcano The results of the zircon U-Pb dating and Hf isotopic data from the CM sequence of Chimei volcano have previously been reported by Shao et al. (2015).

Volcanic History of the CR
Our zircon U-Pb data can be used to identify the stages of volcanism in the CR.According to the U-Pb ages of individual volcanic sequences, the evolution of each volcano over time differed between these four major volcanic  islands.In addition, zircon ε Hf (T) values provided clues that could be used to trace the similarities and differences in the petrogenesis of each volcano.
In Yuemei volcano, the main body of the volcano has already sunk beneath the surface with subduction (Lai and Song 2013;Lai et al. 2017).Consequently, we only observed the SM and STP sequences in the distal volcanic facies of this volcano.Almost all of the zircons in these two sequences are inherited, which indicates that these magmas erupted quickly after they were formed and thus did not exist for long enough to form magmatic zircons.The youngest mean age peak in the STP sequence, which is composed of 4 dates, is 15.5 ± 2.1 Ma; however, this sequence may exhibit an unconformity with the sedimentary sequence of the Fanshuliao Formation, which contains planktic foraminifers with ages of 5.2 -3.35 Ma (Huang et al. 1988).If so, this indicates that an age gap of ~10 Ma exists in the geological record.In addition, the K-Ar and 40 Ar/ 39 Ar ages of the volcanic breccias in the STP sequence in this volcano are 6.2 ± 0.6 Ma (Song 1990) and 7.2 ± 0.1 Ma (Lai et al. 2017), respectively (Table 2).Based on these results, the age of 15.5 Ma might not represent the age of the magmatic zircons in Yuemei volcano.These inherited zircons have high and positive ε Hf (T) values (ranging from +31.4 to +23.3, Fig. 9a), indicating that they may have been picked up from a DM source during the later ascent of magma.
In Chimei volcano, zircons in these volcanic sequences have Oligocene to Miocene U-Pb age peaks of 24.1 ± 1.4 Ma [ε Hf (T) values = +18.0 to +5.7], 14.3 ± 0.5 Ma [ε Hf (T) values = +19.5 to +13.3], and 9.1 ± 0.5 Ma [ε Hf (T) values = +19.9 to +12.5] in the CM sequence (Shao et al. 2015).Only one age data point of 7.9 ± 0.7 Ma was obtained from the SM sequence, and an age of 4.2 ± 0.1 Ma [ε Hf (T) values = +18.9 to +10.0] was obtained from the STP sequence (Fig. 9b and Table 2).Shao et al. (2015) proposed that the age of the oldest igneous exposure in the NLA is ~9 Ma, and they argued that the age of 14 Ma recorded in the CM sequence represents the initiation of the subduction of the South China Sea Plate.Although the previous dating results of this sequence have yielded zircon fission track ages ranging from 16.4 -8.3 Ma (Yang et al. 1995) and whole-rock K-Ar ages ranging from 22.2 -9.0 Ma (Ho 1969;Juang and Bellon 1984;Richard et al. 1986), these data are considered to be inaccurate, due to the occurrence of low-temperature alteration (Lo et al. 1994) or the inheritance of excess radiogenic Ar in minerals or whole-rock samples, which have probably caused these dating results to be older than their erupted ages (Yang et al. 1995).On the other hand, the 206 Pb/ 238 U age result of 4.2 ± 0.1 Ma obtained from the STP sequence is consistent with those of previous studies, such as those of whole-rock 40 Ar/ 39 Ar dating (4.2 ± 0.1 Ma, Lai et al. 2017), whole-rock K-Ar dating (4.4 ± 0.2 Ma, Richard et al. 1986), and zircon fission track dating (4.4 ± 0.6 Ma, Yang et al. 1995).In addition, the Kangkou Limestone, which overlaps the STP sequence, contains planktic foraminifers with ages of 5.2 -3.75 Ma (Huang et al. 1988(Huang et al. , 1995)); these ages also match the time at which volcanism ended.We argue that this age, which was obtained from the STP sequence, represents the timing of the youngest period of magmatism in the Chimei volcano.Moreover, this age is also the youngest U-Pb age obtained from magmatic zircons in the four major volcanoes in the CR.Additionally, magmatic zircons in the STP sequence are larger than those in the older sequences in Chimei volcano; this indicates that this magma remained in the chamber for a long period of time after it was formed.
In Chengkuang'ao volcano, all of the Miocene zircon U-Pb age peaks in volcanic sequences occur within a short interval, from lava flow of the SM sequence [8.3 ± 0.3 Ma, ε Hf (T) values = +24.9 to +16.4], to volcanic breccia of the SM sequence [7.3 ± 0.2 Ma, ε Hf (T) values = +20.2 to +13.6] to the STP sequence [7.0 ± 0.2 Ma, ε Hf (T) values = +22.0 to +4.8] (Fig. 9c).These results imply that the volcanic sequences were rapidly emplaced on this volcanic island.In lava flow of the SM sequence, the oldest age of a single zircon grain is 9.2 ± 0.4 Ma, which indicates that the volcanism here became violent at the same time as it did at Chimei volcano.Previous studies of this sequence here have obtained an age of ~15.2 Ma (K-Ar dating; Richard et al. 1986), which may represent the timing of the initiation of subduction.Some further studies have reported whole-rock 40 Ar/ 39 Ar ages in the STP sequence of 7.4 ± 0.1, 6.2 ± 0.1 Ma (Table 2) (Lai et al. 2017), and 5.4 ± 0.5 Ma (Lo et al. 1994).These results demonstrate that the ages of the STP sequence in this volcano range from ~7.4 -5.4 Ma and that it was then covered by the Tungho Limestone (containing planktic foraminifers with ages of 5.2 -2.9 Ma, Huang et al. 1988Huang et al. , 1995)).
In Tuluanshan volcano, only one sample from the STP sequence shows a U-Pb age peak of 8.5 ± 0.2 Ma (Table 2).This volcano is only exposed at the top of the volcanic sequence.Based on our data, the time at which its volcanism ceased may be ~8.5 Ma.Moreover, the few inherited zircons present in Chengkuang'ao and Tuluanshan volcanoes indicate that small degrees of crustal contamination occurred in these two volcanoes (Fig. 9c).

Implications for the older Zircon
Ages from the NLA 6.3.1 Cenozoic Zircons (30 -15 Ma) Shao et al. (2015) argued that the initiation of NLA magmatism began during the Middle Miocene [14.3 ± 0.5 Ma, with ε Hf (T) values from +19.5 to +13.3 in the CM sequence of Chimei volcano] (Fig. 9b).We also found some zircons with a similar age peak at 15.5 ± 2.1 Ma (n = 3), with very high and positive ε Hf (T) values ranging from +31.4 to +22.8, in the STP sequence of Yuemei volcano (Figs.3b and 9a).Although most of the zircon ages in the Lanyu volcanic island are very young, we found one zircon with an age of 14.8 ± 1.2 Ma and an ε Hf (T) value of +20.0 (Fig. 9d) (Shao et al. 2014).In our data, two other Oligocene age peaks are present in Chimei volcano: one is at 24.1 ± 1.4 Ma and records ε Hf (T) values ranging from +18.0 to +5.7 in the CM sequence, and the other is at 25.7 Ma and has ε Hf (T) values ranging from 0 to -3.0 in the STP sequence (Fig. 9b).
Previous studies have indicated that the South China Sea opened from 32 -15 Ma (Taylor and Hayes 1983).Before the formation of the South China Sea Plate, another old plate, named the proto-South China Sea Plate, might have existed between the South China Block and Kalimantan at ca. 60 Ma (Parker and Gealey 1985;Lee and Lawver 1994).The proto-South China Sea Plate subducted toward the east beneath the Kalimantan and West Philippine Basin from 50 -20 Ma (Williams et al. 1998;Hall 2002).Based on the aforementioned tectonic activities, the initiation of the subduction of the South China Sea Plate might be later than ~20 Ma.
We argue that the ages of 15.5 and 14.8 Ma can also refer to the earlier magmatism of the initiation of subduction of the South China Sea Plate, based on their depleted ε Hf isotopic characteristics [ε Hf (T) values ranging from +31.4 to +22.8] and the fact that these magmas did not erupt extensively until their later eruptions at ~9 Ma.Zircons that record ages of 24 and 25 Ma in the CM sequence of Chimei volcano (24.1 ± 1.4 Ma) have higher ε Hf (T) values (from +18.0 to +5.7) and lower U concentrations (51 -268 ppm) (Shao et al. 2015) than those from the STP sequence [25.7 Ma, ε Hf (T) values from 0 to -3.0, and U contents ranging from 572 -2669 ppm] (Fig. 9b and Tables A1 and A2).However, we only obtained a total of six ages of 25 -24 Ma from these zircons; thus, more evidence is required to understand the exact implications of these ages.

Xenocrystic Zircons of 320 ma
In the STP sequence of Chimei volcano, we not only found the youngest magmatic zircons (4.2 ± 0.1 Ma, sample no.130812-3) (Figs.4e and 8c) but also found an age peak of 319.6 ± 2.3 Ma, with ε Hf (T) values ranging from +19.5 to +3.2 (sample no.STPW, Figs.4i and 8c).This group is unique because these ages are abnormal in the Cathaysia Block.Only one previous study reported similar ages of inherited zircons (322 -314 Ma, n = 13) from two sedimentary rocks recording Permian and Jurassic ages (Hu et al. 2012).However, the Carboniferous zircons in their data have negative ε Hf (T) values (from -5.1 to -18.4), which are obviously different than the positive values exhibited in our zircons.Determining where and how the magma from the Chimei volcano could obtain such inherited zircons with homogeneous ages and positive ε Hf (T) values requires more evidence.

Pre-Miocene Volcanism (> 30 Ma)
Shao et al. (2015) suggested that a micro-continent or a continental fragment split off from the Eurasian margin by the opening of the South China Sea, which then drifted and accreted to the western Philippine Sea Plate prior to the initiation of the subduction of the NLA.This micro-continent or continental fragment was referred to as part of the Cathaysia Block, which belonged to the South China Block in the eastern margin of the Eurasian Plate (Li et al. 2003).According to the zircon dating work performed by Li et al. (2014), a total of 4041 Precambrian detrital zircons from the Cathaysia Block have three major peak ages at ~2485 Ma (the Wutai orogeny, 2600 -2400 Ma), ~1853 Ma (the Luliang orogeny, 1900 -1700 Ma), and ~970 Ma (the Sibao orogeny, 1000 -930 Ma), as well as four subordinate peaks at ~1426, ~1074, ~780 Ma (the Jinning orogeny, 850 -700 Ma), and ~588 Ma (the gray columns in Figs. 8 and 9).In addition to exhibiting Precambrian ages, the Cathaysia Block also records Paleozoic to Mesozoic U-Pb ages, including peaks at 450 -410 Ma (the Caledonian orogeny, Wang et al. 2007) 9d; data from Shao et al. 2014).In addition to these ages, our analyzed results in Yuemei and Lanyu volcanoes and those of another study performed in Chimei volcano (Shao et al. 2015) also indicate that the Cathaysian-type inherited zircons have variable zircon Hf isotopic ratios, in contrast to the magmatic zircons, which record high and positive ε Hf (T) values since the Miocene (Figs. 9b -d).
These Cathaysian-type inherited zircons could have been involved in NLA magmatism by one of two major mechanisms.The first possible way is that the sediments that were eroded from the Cathaysia Block and deposited in the South China Sea Plate were subducted with the subducting slab into the subduction zone.However, when the NLA started to form on the Philippine Sea Plate during the Middle Miocene (~20 -15 Ma), it was still located far away from the Cathaysia continental block, and it thus would have been hard to obtain detrital zircons from this region (Hall 2002).Thus, the subducted oceanic crust was covered by more pelagic sediments (Fourcade et al. 1994;Marini et al. 2005) than continental sediments (Defant et al. 1990;Marini et al. 2005).Additionally, according to our measured zircon Hf isotopic ratios and whole-rock Nd and Hf isotopic compositions (Lai et al. unpublished data), source contamination did not play an important role from 10 -6 Ma (Fig. 10).
Another mechanism for the source of the Cathaysiantype inherited zircons was that magma traveled through a continental fragment prior to being erupted.Smyth et al. (2007) proposed that the Early Cenozoic East Java arc was underlain by a continental fragment of Gondwana, based on their similar inherited zircon ages.Similarly, Shao et al. (2015) suggested that a mirco-continent or a continental fragment of Cathaysia must have overlain the Luzon subduction zone and that old Cathaysian-type inherited zircons were picked up during the ascent of magma to the magma chamber.In addition, several previous studies reported that the North Palawan (Suggate et al. 2014) and Mindoro (Knittel et al. 2010) Continental Terranes were part of the continental margin of the South China Block, which rifted during the opening of South China Sea and were then accreted to the Philippine Sea Plate.According to the results of age and Hf isotopic analyses, we argue the Cathaysian accreted micro-continent or continental fragment was not a unique feature of the NLA in Chimei volcano, but also existed in some volcanoes around this subduction area.

Implications for the Petrogenesis of the NLA
In general, crustal materials can contaminate magmas in one of two main ways: one, source contamination, in which sediments eroded from the continental crust are subducted into the mantle wedge and infect the magma source; and two, crustal contamination, which occurs when magma rises throughout the overlying continental crust and reacts with crustal materials.Based on our data, we argue that magmas in the NLA went through both source and crustal contamination, but to different degrees, prior to being erupted.

evidence of Crustal Contamination
Numerous previous studies have considered crustal contamination when discussing the petrogenesis of the NLA, based on the observed variations in trace elements (Defant et al. 1990), the petrography of xenoliths and minerals (Yang 1992), inherited zircons (Shao et al. 2015), and a primary 143 Nd/ 144 Nd and 87 Sr/ 86 Sr mixing model (Lai et al. 2017).In our study, high proportions of inherited zircons were found in Yuemei, Chimei, and Lanyu volcanoes.These inherited zircons may not have been supplied from the subduction of crustal sediments, because the subduction zone was still located far from the Eurasian continent at 15 -9 Ma (Lee and Lawver 1995;Hall 2002).Moreover, temperatures under intra-oceanic subduction zones can reach higher than 1000°C (Blatt et al. 2006), and the closure temperature of the Pb-U system in zircon is ~900°C (Lee et al. 1997); thus, few zircons could still record inherited Hf isotope ratios after being subducted with the downgoing slab.Our dating results indicate that the ages of the inherited zircons overlap with those of the Cathaysia Block, as discussed above.Based on the aforementioned lines of discussion, we suggest that these inherited zircons were collected from the overlying microcontinent or continental fragment by the ascending magma.Consequently, crustal contamination is an important process in the petrogenesis of the NLA.

evidence of Source Contamination
Previous studies have suggested that source contamination is the main mechanism of the petrogenesis of the NLA (Chen et al. 1990;McDermott et al. 1993;Fourcade et al. 1994;Marini et al. 2005) and have provided several mixing models between a DM component and another enriched component.Our zircon ε Hf (T) values have a decreasing trend of depleted-mantle involvement through time in each volcano in the NLA (Fig. 10a).Magma sources exhibit the characteristics of a DM component, such as an intra-oceanic arc, i.e., the highest ε Hf (T) values recorded in Chimei, Chengkuang'ao, Lanyu, and Lutao after 9 Ma are +19.9,+24.9, +21.3, and +18.1, respectively (Table A2).The zircon ε Hf (T) values of the depleted component are homogeneous before 6 Ma and decrease smoothly after 6 Ma, which is also the time when the arc-continent collision began (Teng 1990).Similar decreasing curves can also be found on the diagrams of whole-rock ε Hf (T) and ε Nd (T) val-ues versus age (Figs.10c and d).Moreover, the U contents of these dated zircons show smoothly increasing trends after 6 Ma (Fig. 10b).Because uranium is an incompatible element, it is enriched in magmas with higher degrees of partial melting, such as those of the continental crust.For this reason, when crustal materials eroded from continental crust are subducted with the subducting slab into the magma source, its U concentrations will increase.
Further evidence can be found based on the variations in zircon Hf isotopic data from each volcano in the NLA.The zircon ε Hf (T) values in the NLA show high and positive from ~15 -8 Ma (+25 to +15; ±5 ε-unit variation), and became lower from ~6 -4.2 Ma (+20 to +8; ±6 ε-units), and the lowest from ~1.3 Ma (+19 to +5; ±7 ε-units) (Fig. 10a).These results can be interpreted as evidence of the binary mixing of magmas between a DM source and the increasing continental crust materials that was subducted into the magma source (Lai et al. 2017).
These results indicate that source contamination occurred in the NLA and may have been due to the addition of crustal materials to the magma source.The fact that the degree of source contamination obviously increases after 6 Ma might be the result of more crustal materials being involved in the subduction zone when the NLA started colliding the Eurasian continental margin.

CoNCLUdING RemARkS
The results of the U-Pb dating and Hf isotopic analysis of zircons from volcanic rocks from six volcanoes of the NLA display various age spectra, implying that their magmatic and tectonic histories are as follows.The 15 -14 Ma peak ages in the CR represent the magmatism caused by the initiation of the subduction of the South China Sea Plate.The main magmatism of the NLA began ca. 9 Ma and ceased ca. 4 Ma in the CR.The magmatism of the NLA erupted after Miocene and showed high and positive ε Hf (T) values.The inherited zircons from Yuemei, Chimei, and Lanyu volcanoes indicate that the magmas in these volcanoes interacted with old zircons from fragments of Cathaysia and thus represent crustal contamination.Our results indicate that these old micro-continents or continental fragments commonly existed around the Luzon Arc.Both crustal and source contamination can be recognized based on the ε Hf (T) values and U concentrations of zircons, as well as the Hf and Nd isotopic compositions of whole-rock samples.The degree of source contamination increases after 6 Ma, which might be the result of arc-continent collision causing the tectonic environment to become more compressive, thus extending the retention time of magmas in chambers; or due to the increasing amount of crustal materials involved in the magma source when the NLA started colliding the Eurasian continental margin.

Fig. 1 .
Fig. 1.Simplified tectonic map of the NLA and geological map of the CR.(a) The NLA includes the CR of Taiwan and two oceanic volcanic islands (i.e., Lutao and Lanyu volcanoes).(b) Volcanic sequences marked by different colors correspond to the four major volcanoes in the CR.Three rectangles represent the ranges of Figs.2a -c.
Fig. 3. (a) U-Pb concordia diragrams of sample no.LD from the SM sequence and no.130811-6 from the STP sequence in Yuemei volcano.The numeric (N) denotes the number of zircon.(b) Weighted mean 206 Pb/ 238 U ages and (c) CL images of magmatic zircons from the STP sequence in Yuemei volcano.
Fig. 4. U-Pb concordia diagrams and CL images of zircons from the CM, SM, and STP sequences in Chimei volcano.(a) Ages from the CM sequences were published by Shao et al. (2015).(b) Sample no.SM-1 and STP-1 from SM and STP sequences, respectively.(c) and (d) Sample no.CM-10 from CM sequence.(e) and (f) Sample no.130812-3 from STP sequence.(g) to (i) Sample no.STPW from STP sequence.

Fig. 9 .
Fig. 9. Plot of zircon ε Hf (T) values versus crystallization ages from six volcanoes in the NLA.(a) Yuemei volcano, (b) Chimei volcano, (c) Chengkuang'ao and Tuluanshan volcanoes, (d) Lutao and Lanyu volcanoes.Data from the CM sequences in the Chimei volcano were published by Shao et al. (2015).Lutao and Lanyu zircon U-Pb ages were published by Shao et al. (2014), and their Hf isotopic analyses were performed in this study.Data from different volcanic sequences are marked with different symbols.The gray areas show the Cathaysian-type ages (data from Wanget al. 2005, 2007;Chen et al. 2008;Li et al. 2014).The orange areas show Hf isotopes of the Cathaysian-type zircons (data fromLi et al. 2014).
Fig. 10.(a) and (b) Plot of zircon ε Hf (T) values and uranium concentration versus crystallization age after 16 Ma.The dotted pink and blue arrows represent the degree of source contamination, which increases after 6 Ma.(c) and (d) Plots of whole-rock ε Hf (T) and ε Nd (T) values (Lai et al. unpublished data) versus their mean U-Pb ages.The trend of source contamination can also be observed in the whole-rock patterns.Data from the CM sequences in the Chimei volcano were published by Shao et al. (2015).
Fig. A1.CL images of zircons from the CR.(a) Sample no.LD from the STP sequence in Yuemei volcano.(b) Sample no.SM-1 from the SM sequence in Chimei volcano.(c) Sample no.STP-1 from the STP sequence in Chimei volcano.

Table 2 .
Ages of the volcanic sequences in volcanoes of the Northern Luzon Arc.
Chen et al. 2008)e Indosinian orogeny,Wang et al. 2005), and 180 -65 Ma (the Yanshanian orogeny,Chen et al. 2008) (the gray columns in Figs.8 and 9).Inherited zircons dated to the Precambrian, Paleozoic and Mesozoic collected from several volcanoes in the NLA show similar age peaks as those detrital zircons from the Cathaysia Block, such as those from Yuemei volcano, Chimei volcano, and Lanyu volcano.Yuemei volcano records in-

Table A1 .
Zircon U-Pb isotopic data of Coastal Range samples.

Table A2 .
Zircon Hf isotopic data of the Northern Luzon Arc.