Origin of Mafic Microgranular Enclaves (MMEs) and Their Host Rocks of the Cretaceous Xiaojiang-Liangnong Granitic Complexes in the Southeast Coast Magmatic Belt, S China


Cretaceous Late Yanshanian (LY) magmatism produced large amounts of I-type granitoids in the Southeast Coast Mag matic Belt (SCMB). I-type granitoids from Xiaojiang-Liangnong complexes (N Zhejiang) in the northern part of this belt are characterized by containing abundant mafic microgranular enclaves (MMEs). On the basis of petrography and mineralogy, two types of MMEs are recognized. One (Type 1), enclosed in the granodiorite, is composed of amphibole, high-Ti tabularbiotite, plagioclase, K-feld spar, and quartz. Us ing the Al-in-am phibole geobarometer, a shallow origin (4 - 8 km) of am phiboles is suggested. The other (Type 2), enclosed in al kali feld spargranite, is amphibole-free and composed of low-Ti acicular biotite, plagioclase, K-feld spar, and quartz. Geo chemistry of major and traceel ements seems to re flect two different evolving trends for these host granitoids. The Sr-Nd isotopic features indicate that all the studied samples vary in a narrow range of Isr (0.7078 to 0.7082) and eNd(T) (-5.6 to -8.3), except the Type 2 enclaves and hosts that show slightly morede pleted compositions (Isr = 0.7073 to 0.7075 and eNd(T) = -5.0 to -6.0). U-Pb zir con ages obtained are 109.6 ¡_ 0.9 Ma for the granodiorite and 113.5 ¡_ 1.1 Ma for the alkalifeld spargran ite, with 103.2 ¡_ 0.9 Ma for the Type 2 MMEs. In her ited ages of 117.0 ¡_ 2.0 Ma and 119.8 ¡_ 1.1 Ma seem to presentin the former two samples. Similarities of mineral chemistry and isotopic composition among the MMEs-host pairs as well as the youn ger age of MMEs suggest that MMEs of Xiaojiang-Liangnong complexes most likely represent mixing/mingling products formed as a consequence of the in tru sion of ba saltic magma into the host granitoids un der dif fer ent cool ing con di tions. Judg ing from the pres ence of in her ited zir con ages, the com po si tion gap be tween high-Ti and low-Ti bi o tite in granodiorite and al kali feld spar gran ite, and lower zirconsaturation temperatures for the host granitoids (cal cu lated as 787¢XC and 763¢XC, re spec tively, for these two rock types), or i gin of the LY I-type granitoids in the Xiaojiang-Liangnong com plexes is re lated to par tial melt ing of dif fer ent sources, with the preceding alkali feldspar granite derived from more quartzofeldspathic and later granodiorite, from more mafic sources.

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