北大别山牢山寨岩体地球化学特征和LA-ICP-MS锆石U-Pb年龄

对北大别山牢山寨岩体进行了锆石U-Pb定年、岩石全岩主量和微量元素分析,发现牢山寨岩体二长花岗岩在化学成分上具超酸、富碱、过铝质等特点,属超酸性富碱高钾钙碱性系列;大离子亲石元素Rb、K富集,高场强元素Hf、Th、U与轻稀土元素明显富集,重稀土元素亏损,具中等负Eu异常等特征,稀土元素配分曲线为左高右平缓的轻稀土元素富集型。锆石LA-ICP-MSU-Pb定年测试得到岩体~(206)Pb/~(238)U加权平均年龄值为133±2 Ma(n=21,MSWD=3.1),代表牢山寨岩体的侵位年龄,表明其为早白垩世的岩浆活动产物。推测其形成机制为:在秦岭-大别造山带中生代构造体制转换及其后伸展机制下,下地壳尚未发生拆沉,含斜长石+辉石+角闪石+石榴石的高压麻粒岩下地壳部分熔融,沿深断裂向上运移,在地壳浅部固结成岩形成牢山寨岩体。

Geochemical characteristics and LA-ICP-MS zircon U-Pb dating of Laoshanzhai rock mass in North Dabie Mountain

On the basis of zircon U-Pb dating and rock total analysis of Laoshanzhai rock mass, the authors hold that chemical components of Laoshanzhai rock mass have ultra-acid, alkali-rich and peraluminous characteristics, thus belonging to ultra-acid, alkali-rich and high-K calc-alkali series. The data indicate that Laoshanzhai rock mass is characterized by enrichment of LILE (Rb, K), HFSE (Hf, Th, U,) and LREE, and depletion of HREE, with moderate negative Eu anomaly. Rare earth element distribution curve is higher on the right side and flat on the left side, exhibiting light REE enrichment. Major trace elements show that magma of rock mass was mainly from lower crust source, and was probably mixed with a small amount of mantle materials. Zircon LA-ICP-MS U-Pb dating shows that ²⁰⁶Pb/²³⁸U weighted average age value of rock mass is 133±2 Ma (n=21, MSWD=3.1), which represents the emplacement age of Laoshanzhai rock mass as the product of magmatic activities of Early Cretaceous period. Laoshanzhai rock mass was formed by tectonic regime transition and the extension mechanism of Qinling-Dabie orogenic belt in Mesozoic, during which the lower crust had not yet been delaminated, the lower crust of high pressure granulite that contained plagioclase, pyroxene, hornblende and garnet was subjected to partial melting, and experienced upward migration along deep faults, then experienced consolidation diagenesis in the shallow crust.