西藏申扎早白垩世花岗岩类:板片断离的证据

Early Cretaceous granitoids in Xainza, Tibet: Evidence of slab break-off

拉萨地块中北部地区在~113Ma发生的带状岩浆大爆发事件被认为是南向俯冲的班公湖-怒江洋壳岩石圈板片断离的结果,但缺乏更多的地球化学数据。本文报道的拉萨地块中部申扎地区花岗岩类地球化学数据为这一模型提供了可靠的地球化学证据。申扎花岗岩类包括寄主花岗闪长岩(113Ma),辉长质包体和闪长质包体(111Ma)。申扎早白垩世花岗岩中的寄主岩石与镁铁质包体均属于偏铝质中-高钾钙碱性系列,所有样品均富集大离子亲石元素(如Rb、Ba、U和Th)、亏损高场强元素(如Nb、Ta、Ti和Zr)。花岗闪长质寄主岩和闪长质包体具有高的(87Sr/86Sr)i值(0·7121~0·7133),负的εNd(t)值(-10·2^-9·1)及古老的Nd同位素模式年龄(1·90~1·55Ga);辉长质包体具偏低的(87Sr/86Sr)i值(0·7067和0·7108),较高的全岩εNd(t)(-2·1和-4·1)值,Nd同位素模式年龄为1·33Ga和1·32Ga。申扎花岗岩类中的辉长质包体具有板内玄武岩亲缘性,这一特征与班公湖-怒江洋壳岩石圈南向俯冲板片在~113Ma左右发生的板片断离有关。申扎花岗岩类是在板片断离背景下,主要由来源于古老拉萨基底物质重熔的熔体与上涌的软流圈地幔物质发生岩浆混合产生的。

The magmatic flare-up at ~113Ma occurred in the central and northern Lhasa Terrane has been attributed to the slab break-off of the southward subduction of the Bangong-Nujiang Ocean seafloor, but the bulk-rock geochemical data are limited. Bulk-rock data, including major element, trace element, and Sr-Nd isotopic data of the granitoids from Xainza in the central Lhasa Subterrane, Tibet, reported in this paper provide reliable geochemical lines of evidence for this model. The granitoids from Xainza include granodiorite （113Ma）, gabbroic enclaves and dioritic enclaves （111Ma）.The host granitoids and mafic enclaves are all metaluminous, medium- to high-K calc-alkaline, and are enriched in LILE （such as Rb, Ba, U and Th） and depleted in HFSE （such as Nb, Ta, Ti, Zr）. The host granodiorites and dioritic enclaves have higher initial 87Sr/86Sr values （0.7121~0.7133）, lower and negative εNd（t） values （-10.2^-9.1）, and older Nd isotopic model ages （1.90~1.55Ga） compare to gabbroic enclaves that display lower initial 87Sr/86Sr values （0.7067 and 0.7108） and higher εNd（t） values （-2.1 and -4.1） with Nd isotopic model ages of 1.33~1.32Ga. The gabbroic enclaves from Xainza exhibit geochemical affinity of within-plate basalts, which can be attributed to the slab break-off at that time. We propose that the Xainza granitoids are derived largely from anatexis of the ancient Lhasa basement that mixed with rising asthenosphere materials in response to the slab break off of the southward subduction of the Bangong-Nujiang Ocean seafloor.