胶西北秦姑庵花岗岩体岩石成因及对金成矿的启示：元素地球化学、锆石U—Pb年代学及Lu—Hf同位素制约

Petrogenesis of the Qingu’an pluton from the northwestern Jiaodong Peninsula and its implications for gold mineralization: Constraints from petrogeochemistry, zircon UPb age and LuHf isotopes

摘要:

本文通过对分布于胶西北地区南部的秦姑庵花岗岩体开展详细的岩石学、岩石地球化学、锆石UPb年代学和锆石LuHf同位素地球化学研究，识别出粗中粒二长花岗岩、斑状中粒二长花岗岩和细粒黑云母二长花岗岩，它们具有相同的地球化学特征和同位素组成，呈连续演化关系，为同源岩浆活动的产物。岩石地球化学研究表明，花岗岩具有高的SiO2（68.73％～75.27％）、全碱（K2O+Na2O=8.39％～8.93％），低的Al2O3（13.38％～15.84％）、MgO（0.14％～0.98%）、P2O5（0.02％～0.13%）、Sr（118×10-6～520×10-6）、Y（7.03×10-6～12.30×10-6）、Yb（0.78×10-6～1.34×10-6）含量特征，轻稀土（LREE）富集、重稀土（HREE）相对亏损（LREE／HREE=18.39～26.72），轻、重稀土元素分馏明显［(La/Yb)N＝17.13～42.58］，Eu中等负异常（δEu=0.49～0.76），富集大离子亲石元素（LILE），亏损高场强元素（HFSE）。锆石SHRIMP定年结果表明，斑状中粒二长花岗岩形成于（119.9±1.3）Ma，秦姑庵岩体形成于早白垩世晚期，时代上与胶东地区伟德山型花岗岩一致。锆石原位LuHf同位素分析表明，εHf（t）为24.07～19.27，二阶段模式年龄TDM2为2398～2696 Ma，岩浆源区主要来源于古元古代物质的部分熔融，少部分为新太古代基底岩石。岩体形成于加厚下地壳的下部，其残留相可能由斜长石+辉石+角闪石+石榴石组成，相当于麻粒岩相或石榴石麻粒岩相。花岗岩的同位素年龄与胶西北金矿床的成矿时代一致，指示120Ma左右的岩浆活动为金成矿提供了流体和热动力条件。

Abstract:

In this paper, through study on petrography, petrogeochemistry, zircon UPb chronology and LuHf isotopic geochemistry of Qingu'an granite in northwestern Jiaodong Peninsula, coarse and medium grained monzogranite, porphyritic medium grained monzogranite and fine biotite grained monzogranite have been identified. They have the same geochemical characteristics and isotopic compositions and present continuous evolution tendency.They are the product of the same magmatic activity. As showed by the petrogeochemistry study, the granites have high SiO2 (68.73%~75.27%), total alkali (K2O+Na2O=8.39%~8.93%),with low Al2O3(13.38%~15.84%),MgO(0.14%~0.98%),P2O5 (0.02%~0.13%),Sr (118×10-6 ~ 520×10-6), Y (7.03×10-6 ~ 12.30×10-6), Yb(0.78×10-6 ~ 1.34×10-6),enrichment in LREEs and relative depletion in HREEs (LREE/HREE=18.39~26.72), the fractionation distillation of the LREEs and HREEs is obvious［(La/Yb)N=17.13~42.58, Eu is moderately negative abnormal（δEu=0.49~0.76).The LILE is enriched, while the HFSE is depleted.The zircon SHRIMP UPb dating yield （119.9±1.3）Ma for the porphyritic medium grained monzogranite. It is Indicated that they are formed in early Cretaceous,and belong to Weideshan type granite in Jiaodong area. As showed by LuHf isotope analysis of zircon,εHf(t) is 24.07~19.27,and TDM2 is 2398~2696Ma. It is indicated that the partial melting of Paleoproterozoic materials and few Neoarchean basement rocks are involved in the primordial magma. The above data analysis shows that Qingu'an granite might be generated from the lower part of the thickened lower crust, and its residual phase may be composed of plagioclase+pyroxene+amphibole+garnet,which is equivalent to granulite facies or garnet granulite facies.The isotopic age of granite is consistent with gold deposit in northwest Jiaodong Peninsula. It is indicated that magmatic activity around 120Ma will provide fluid and thermal dynamic conditions for gold mineralization.