藏北阿翁错地区中基性脉岩年代学、地球化学特征及其板内伸展构造作用

班公湖一怒江缝合带在中生代的构造岩浆演化是青藏高原基础地质研究的热点之一。在该缝合带南部的北冈底斯地区发育有大量的中生代火山岩及相关岩浆岩,其岩石成因与成岩地球动力学背景长期以来存在较大的争议。本文以班公湖一怒江缝合带西段阿翁错地区的中基性脉岩为研究对象,对该地区广泛发育的脉岩的形成时代、岩石成因、构造环境以及动力学背景进行了探讨。研究区辉绿岩脉以低硅(SiO_2=50.03%～51.13%)、低铝(Al_2O_3=15.52%～16.03%、低钛(TiO_2=1.22%-1.31%)、富镁(MgO=9.19%～10.37%)、富钠(Na_2O=3.10%～3.58%,Na_2O/K_2O=1.73～1.87)为特征;闪长岩脉以相对高硅(SiO_2=55.58%～56.22%)、低镁(MgO=4.69%～4.64%)、低钛(TiO_2=1.01%～1.06%)、高铝(Al_2O_3=17.74%～18.72%)、富钠(Na_2O=1.55%-5.03%,Na_2O/K_2O=1.81～3.61)为特征,属于钠质低钾钙碱性系列岩石。二者轻重稀土元素分馏明显,均表现为右倾型,具弱的Eu负异常,重稀土元素总体上变化不大。微量元素上均表现出以富集Rb、U等大离子亲石元素和轻稀土(LREE)元素,亏损Nb、Ta、Ti高场强元素(HFSE)为特征。年代学分析结果显示研究区闪长岩脉LA-ICP-MS锆石U-Pb年龄为(99.2±1.2) Ma,辉绿岩脉LA-ICP-MS锆石UPb年龄为(108.4±2.9) Ma,属于早白垩世晚期向晚白垩世过渡期间,其岩浆源区遭受了一定程度的地壳物质的混染,后经历了不同程度的铁镁矿物和斜长石的结晶分离作用。构造环境分析显示研究区中基性岩脉形成于伸展构造环境下,其产状受区域应力场控制,其动力学背景可能与班公湖一怒江特提斯洋南向俯冲消减过程中板片断离导致的软流圈上涌,诱发岛弧和岛弧后方的地幔岩浆发生补充性对流循环而形成的伸展构造背景有关。表明研究区至少在99.2 Ma时,班公湖—怒江特提斯洋俯冲消减已经完成,由早期构造挤压环境转向晚期构造伸展环境。

Geochemistry and geochronotry of intermediate-basic dikes in Awengcuo area of north Tibet and intraplate extensional structures

The Bangong Co-Nujiang suture zone and its tectonic-magmatic evolution constitute one of the hottest scientific problems related to fundamental geology of the Tibetan Plateau. The Mesozoic volcanic-intrusive rocks are widely distributed in the north Gangdese belt, which is also located in southern Bangong Co-Nujiang suture zone. The petrogenesis and geodynamic setting of those rocks remain controversial. In this paper, the authors reported the newly found intermediate-basic dikes in Awengcuo area which is located at the west segment of Bangong Co-Nujiang suture zone so as to explore these problems, and detailed LA-ICP-MS zircon U-Pb dating geochronological and element geochemical studies were carried out for the intermediatebasic dikes. The basic dikes exhibit SiO₂ content of 50.03%-51.13%, Al₂O₃ content of 15.52%-16.03% with TiO₂ content of 1.22%-1.31%, MgO content of 6.12%-8.51%, Na₂O content of 3.10%-3.58%, with Na₂O/K₂O ratio of 1.73-1.87. The diorite dikes have values of SiO₂ (55.58%-56.22%), MgO (4.69%-4.64%), Al₂O₃ (1.01%-1.06%) with TiO₂ (1.01%-1.06%), Na₂O (1.55%-5.03%), and Na₂O/K₂O (1.81-3.61). The diabase veins belong to alkaline basalt series and the diorites are subalkaline-series rocks. All of their light rare elements are concentrated evidently and heavy rare elements are deficient with a right dip distribution mode of REE, slightly negative Eu anomalies and content of HREE. On primitive mantle-normalized trace element diagrams, the intermediate-basic dikes display different degrees of enrichment of LIFEs (e.g., Rb, U), relative depletion of HFSE (Nb, Ta, Ti). The zircon U-Pb dating of diorite-dyke yielded a weighted average age of (99.2±1.2)Ma, and the basic-dyke U-Pb dating yielded an age of (108.4±2.9)Ma, indicating that the diorite-dykes in Awengcuo area were formed at the late stage of early Cretaceous. The magma source region was mainly influenced by the crustal material and underwent different degrees of fractionation crystallization of mafic minerals and plagioclases when uplifting from the high magmatic chamber. The intermediate-basic dikes were generated in a contineral intraplate setting, as shown by analyzing tectonic setting, and its attitudes were controlled by the regional tectonic stress field. The tectonic dynamics background of the dikes formed in slab break-off caused asthenosphere upwelling extension during the Bangong Co-Nujiang Tethyan Ocean''s southward subduction at the late stage of early Cretaceous, indicating that the Bangong Co-Nujiang Tethyan Ocean finished the subduction at 99.2 Ma at least, and the regional stress state turned from collision to intraplate extension at the late stage.