西藏南木林晚白垩世辉绿岩与花岗质脉岩成因及其揭示的伸展背景  

Late Cretaceous diabase and granite dike in Namling,Tibet: Petrogenesis and implications for extension

作　　者：叶丽娟[1,2] 赵志丹[1] 刘栋[1] 朱弟成[1] 董国臣[1] 莫宣学[1] 胡兆初[3] 刘勇胜[3] 

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室,地球科学与资源学院,北京100083 [2]天津地质矿产研究所,天津300170 [3]中国地质大学地质过程与矿产资源国家重点实验室,地球科学学院,武汉430074

出　　处：《岩石学报》2015年第5期1298-1312,共15页Acta Petrologica Sinica

基　　金：国家973项目(2011CB403102;2015CB452604);国家自然科学基金项目(41273044;41225006);中国地质调查局工作项目(1212011121260;1212011121066;12120113086400);长江学者创新团队发展计划资助(IRT1083)联合资助

摘　　要：青藏高原南部拉萨地块晚白垩世岩浆岩带的岩石成因和动力学对于揭示大陆碰撞之前的特提斯演化过程具有重要意义。本文报道了南木林县南部的辉绿岩岩株及侵入其中的正长花岗岩脉的锆石U-Pb年龄、Hf同位素和微量元素以及岩石地球化学数据,并探讨了晚白垩世构造-岩浆作用过程。辉绿岩年龄为93.8Ma,正长花岗岩脉年龄为92.4Ma,为同期晚白垩世岩浆活动产物。辉绿岩富集大离子亲石元素,亏损高场强元素,锆石εHf(t)为+8.5^+14.2,具有亏损的岛弧岩浆特征,可能是俯冲交代的地幔楔熔体与软流圈熔体相互作用发生部分熔融的产物;正长花岗岩具有富Si O2(74.76%~76.02%)、富K2O(4.75%~6.18%)、富碱(K2O+Na2O=7.57%~8.44%),以及过铝质特征(A/CNK=1.10~1.18),εHf(t)为+7.7^+13.0,岩石属于高分异的I型花岗岩,可能源于新生的镁铁质下地壳的部分熔融。从南木林地区产出的基性岩和酸性脉岩构成的"双峰式"岩石组合、靠近弧后的伸展构造背景、软流圈和岩石圈的共同参与、上地幔和下地壳同时产生岩浆作用等过程,结合78~100Ma岩浆作用大规模东西向带状分布等研究结果,本文认为南木林南部岩浆岩可能是在新特提斯洋壳北向俯冲挤压的同时,在大约90~100Ma时期,深部发生了板片断离或者板片回转,浅部造成了弧后伸展的构造背景。位于板片窗上方,源于交代地幔楔和软流圈相互作用产生了镁铁质岩浆,上侵后诱发下地壳部分熔融形成花岗质岩浆。基性岩石、花岗质岩石以及特殊类型的紫苏花岗岩、埃达克岩等多种岩石类型并存,表明晚白垩世时期,特提斯洋向着拉萨地块之下的总体俯冲背景下,可能存在洋脊俯冲、板片断离、板片回转等多种构造-岩浆体制。The petrogenesis and geodynamic of Late Cretaceous magmatism from the southern Lhasa Terrane,Tibet,played very important role in discovering the evolution and processes of the Tethyan Ocean before the collision between India and Asia. This paper reports zircon U-Pb age,elements and Hf isotopes,and whole rock geochemical data for the diabase and intruded granite dikes to the south of Namling County,Tibet,and discuss the implications for the Late Cretaceous tectono-magmatism processes in southern Lhasa terrane. The granite dike（ 92. 4Ma） intruded into the diabase pluton（ 93. 8Ma）,which are both products of Late Cretaceous. The Namling diabase is enriched in LILE,depleted in HFSE,with εHf（ t） ranging from ＋ 8. 5 to ＋ 14. 2,which suggests an arc-related rock that probably generated from the partial melting of both lithosphere and asthenosphere mantle. The Namling granites are enriched in Si O2（ 74. 76% ～ 76. 02%）,K2O（ 4. 75% ～ 6. 18%） and K2 O ＋ Na2O（ 7. 57% ～ 8. 44%）. They are peraluminous（ A / CNK =1. 10 ～ 1. 18） high-K calc-alkaline rocks,with εHf（ t） ranging from ＋ 7. 7 to ＋ 13. 0. They are high fractionation I-type granite,which possibly came from the partial melting of a juvenile underplated mafic lower crust. According to the overall features,including bimodal magmatism rock assemblages（ diabase and granite）,back-arc extension environment in the northern margin of the Gangdese batholith belt,interaction between lithosphere and asthenosphere mantle,partial melting from both mantle and lower crust,and the parallel spatial distribution of the Late Cretaceous magmatic rocks with age of 78 ～ 100 Ma,this study employed the slab break-off,or slab rollback models to illustrate the tectono-magmatism processes in southern Tibet. During the subduction of Tethyan Oceans,slab break-off,or slab roll-back occurred in the upper mantle depth during 90 ～ 100 Ma and meanwhile caused the shallow crustal level extension in the back-arc region. Over the slab window of the

关 键 词：晚白垩世 辉绿岩 正长花岗岩脉 伸展作用 南部拉萨地块 西藏 

分 类 号：P588.124[天文地球—岩石学] P588.13[天文地球—地质学]