冈底斯带谷露区中新世花岗岩地球化学特征及构造环境

Geochemistry and Tectonic Setting of Miocene Granitoids Near Gulou Area,Gangdise Belt

谷露花岗岩是冈底斯构造带上念青唐古拉花岗岩的一部分,分布在当雄拉萨大型北北东向伸展断裂两侧。岩石类型主要有斑状花岗闪长岩、巨斑黑云母花岗岩、黑云二长花岗岩、含石榴石花岗岩等。花岗岩同位素K-Ar测年结果在11Ma左右,形成于中新世。花岗岩具有富硅、铝和碱,贫铁、镁、钙的特点,为准铝质高钾钙碱性花岗岩。花岗岩轻稀土富集,重稀土相对亏损,具有较明显的负铕异常。岩石相对富集不相容元素(LILE),贫化高场强元素(HFSE),在原始地幔标准化蛛网图上显示Rb、Th强烈富集,Nb与Ta亏损近似,Ba、Sr和Ti强烈亏损的特点。谷露中新世花岗岩形成于后碰撞构造环境,与该区地壳东西向拉伸阶段的构造环境有关,是碰撞造山后地壳伸展、快速隆升背景下减压深熔的结果。

Located at both sides of the large, NNE trending Damxung-Lhasa extension fault, the Gulou granitoids belong to part of the Nyainqentanglha granites in the Gangdise tectonic belt. The main rock types include porphyritic granodiorite, porphyritic granite, biotite adamellite and garnet-granitite. Their K - Ar dating age are about 11 Ma, revealing that the Gulou granitoids were formed in the Miocene epoch. The Gulou granitoids are high in SiO2, Al2O3 and K2O＋Na2O, and are low in FeO＋ Fe2O3, MgO and CaO, showing the features of metaluminous high-K calc-alkaline granites. They are also LREE-rich, HREE-depletion, have obvious Eu negative anomaly; LILE-rich and HFSE-depletion. In the curves of the trace element distribution patterns normalized by the primitive mantle, they are Rb, Th-rich and Nb, Ta, Ba, Sr, Ti-depletion. These Miocene granitoids were interpreted to be formed in a post-collisional tectonic setting and to have a close relationship with the E--W crustal extension. They were resulted from the crustal partial melting related to crustal extension and plateau uplifting post the collisional orogeny.