北大别大山坑二长花岗片麻岩的地球化学特征与锆石U-Pb年代学

GEOCHEMICAL CHARACTERISTICS AND U-Pb ZIRCON DATING OF DASHANKENG MONZONITIC GRANITIC GNEISS IN NORTHEASTERN DABIE MOUNTAINS

北大别东部大山坑片麻岩主要由钾长石、斜长石和石英组成,少量角闪石、黑云母和褐帘石,成分为二长花岗质。岩石以富碱(Na2O+K2O)尤其是K2O、贫Al2O3为特征,地球化学性质上表现为富集K、Rb、Th、La、Ce等大离子亲石元素而亏损Nb、Ta、U等高场强元素及Sr元素,高的Ga含量、强的负Eu异常(δEu=0.37)和相对较强的轻重稀土分馏程度((La/Yb)N=16.75)。岩石地球化学的总体特征与南大别水吼地区的A型花岗片麻岩类似,意味着其原岩与南大别的A型花岗片麻岩一样,可能也是在拉张状态下形成的一套偏碱性的花岗岩。该二长花岗片麻岩的锆石U Pb年龄为229.2±5.5Ma,也与南大别超高压变质的年龄相似,指示北大别正片麻岩印支期可能也经历过超高压变质作用。南、北大别造山带可能具有相同的形成与演化历史,现南、北大别变质带之间的差异可能更多的是后造山历史不同,尤其是燕山期花岗岩侵入对两个带影响的不同造成的。

The Dashankeng gneiss located in the northeastern Dabie mountains is composed mainly of potassium feldspar, plagioclase and quartz with minor hornblende, biotite and allanite, belonging to monzonitic granitic in composition. The rock is characterized by rich in alkaline (particularly K2O), depleting in Al2O3, rich in largeion lithophile elements (K, Rb, Th, La, Ce), poor in highfield strength elements (Nb, Ta, U) and Sr. The Ga content is high (22μg/g), and the REE pattern of the rock is strongly fractionated ((La/Yb)N=16.75) with prominent negative Eu anomalies(δEu=0.37). The whole geochemical feature of the gneiss is similar to the Atype granitic gneisses in Shuihou area in the southern Dabie mountains, showing that the protolith being also resulted from partial melting of crustal materials existing in the rift zone along the northern margin of the South China block. The UPb dating of zircon from the gneiss is 229.2±5.5Ma, it is similar to the UHP metamorphic ages in the southern Dabie mountains too, showing that the gneisses in the northern Dabie mountains may also had undergone UHP metamorphism in Indosinian. They may have similar history of forming and evolution between southern and northern Dabie belts, and the present differences between the southern and northern Dabie mountains may due to the different history of postorogenic, particularly the different effect of Cretaceous granite intrusions on the two belts.