柴达木地块北缘牛鼻子梁镁铁质-超镁铁质岩体岩石成因与成矿条件

Petrogenesis and mineralization of Niubiziliang mafic-ultramafic intrusion in the northern margin of Qaidam Block,NW China

牛鼻子梁岩体位于柴达木地块西北缘。岩体出露面积约8km2，平面形态呈长条状，长轴方向近东西向。锆石LA-ICP—MSU-Pb年龄为367．0±2．0Ma。岩体中堆晶结构、堆晶韵律和旋回发育，属典型的层状岩体。岩浆分异充分，岩石类型丰富。主要岩石类型有斜长二辉橄榄岩、斜长单辉橄榄岩、角闪二辉橄榄岩、角闪橄榄岩、角闪橄榄二辉岩、角闪二辉岩、橄榄二辉角闪石岩、角闪橄榄辉长岩、细粒辉长岩、似斑状辉长岩、暗色辉长岩、辉长岩、淡色辉长岩、石英闪长岩、英云闪长岩。岩浆源区为高镁拉宽玄武质岩浆（MgO=10．8％），主体岩浆结晶温度为1100-1178℃。岩浆演化过程中主要发生了橄榄石的分离结晶作用，此外还有少量斜长石的分离结晶／堆晶作用。野外地质观察、岩石薄片观察及岩石地球化学特征表明岩体与围岩之间发生了较强的同化混染作用，并且同化混染强度伴随着岩浆演化过程而逐渐增大。大量的同化混染导致岩石化学系列从拉斑玄武质系列转化为钙碱性系列。岩浆源区属亏损型地幔源区。岩体形成的构造环境为大陆边缘裂解环境。从构造环境、原生岩浆、岩体类型、岩浆分异程度、岩浆含水量、同化混染和橄榄石镍含量七个方面来看牛鼻子梁岩体形成镍铜硫化物矿床的潜力很大。

Niubiziliang intrusion is located in northwestern Qaidam Block. It covers an area of about 8km2 and is irregular shaped with east-west extension. Zircon LA-ICP-MS dating is 367.0 ± 2. 0My. The intrusion is a layered intrusion with magmatie layering, laminating, clear cumulate layers and vertical zone. There are abundant types of rocks because of complete magmatic differentiation, and the rocks are plagioelase lherzolite, hornblende lherzolite, wehdite, olivine websterite, olivine gabbre, olivine amphobolite, websterite, gabbro, leucogabbro, quartz diorite and tonalite. The primary magma was high-basalt magma with MgO up to 10. 8% and the crystallization temperature of the main magma was 1100 ~ 1178~C. The mainly fractional crystallized or cumulated phases are olivine and minor plagioelase during magma evolution. Field geological observation, petrography and geochemistry proved that the magma has experienced significant crustal contamination. Furthermore, the degrees of assimilation increased during the magma evolution. Extensive crustal contamination would lead rock chemical series to cale-alkalie series from tholeiitic series. The mantle source was depleted, and its tectonic setting belonged to rifted continental margin. The Niubiziliang intrusion has great potential of forming Ni-Cu sulfide deposits according to tectonic setting, primary magma, rock types, magmatie differentiation, magmatic water content, assimilated contamination and the Ni content of olivine.