西藏北冈底斯巴尔达地区岩浆作用的成因:地球化学、年代学及Sr-Nd-Hf同位素约束

Petrogenesis of magmatism in the Baerda region of Northern Gangdese,Tibet:Constraints from geochemistry,geochronology and Sr-Nd-Hf isotopes

前人对西藏冈底斯中北部白垩纪岩浆岩的岩石成因和地球动力学背景仍然存在很多解释。本文报道了位于北冈底斯(拉萨地块北部)东段巴尔达地区的岩浆岩锆石U-Pb定年、元素地球化学和Sr-Nd-Hf同位素数据,以对这一问题进行约束。巴尔达岩浆岩包括花岗闪长斑岩和安山玢岩。锆石U-Pb定年结果表明,巴尔达岩浆岩侵位于～114Ma,是北冈底斯早白垩世岩浆岩的典型代表之一。巴尔达岩浆岩两种岩石均为高钾钙碱性系列,其中花岗闪长斑岩的SiO2为64.45%～67.52%,全碱(K2O+Na2O)含量为6.23%～7.47%,铝饱和指数(A/CNK)为0.98～1.22,为兼具I型特点的S型花岗岩;安山玢岩SiO2为57.50%～62.15%,K2O含量为2.30%～3.60%,MgO含量为2.27%～4.40%。两种岩石均富集大离子亲石元素和Pb,亏损高场强元素和Ba等。与冈底斯成熟大陆地壳相比,巴尔达岩浆岩具有更高的εNd(t)值(-7.8～-6.8)和相对低的(87Sr/86Sr)t值(0.7094～0.7105),并且具有不均一的锆石εHf(t)值(-4.2～+3.3)和相对年轻的Hf模式年龄(639～975Ma)。锆石Hf同位素数据和锆石饱和温度(768～818℃)均显示幔源物质在巴尔达岩浆岩形成过程中发挥了重要作用。巴尔达岩浆岩可能是在南向俯冲的班公湖-怒江洋岩石圈断离背景下,由上涌的软流圈提供热量诱发岩石圈地幔熔融产生幔源基性熔体侵位到上部地壳,并引发周围酸性物质的熔融混合,混合过程中幔源组分的输入使S型熔体向I型熔体转化。

The petrogenesis and geodynamic setting of Cretaceous magmatic rocks in the middle and northern Gangdese (i.e.,the northern part of the Lhasa Terrane),Tibet remain debated so far. In order to explore these issues,this paper reports the data of bulk-rock major element,trace element,Sr-Nd isotopes and zircon Hf isotopes of the magmatic rocks exposed in the Baerda region of the northern Gangdese,Tibet. These rocks consist of granodiorite porphyry and andesite porphyrite. Zircon U-Pb age dating indicates that they were emplaced at ca. 114Ma,and can be considered as representing one of the Early Cretaceous magmatic rock bodies in the northern Gangdese. All samples in Baerda are high-K calc-alkaline,in which granodiorite porphyry samples that exhibit SiO2 contents of 64.45%~67.52%,total alkali (K2O+Na2O) contents of 6.23%~7.47% with A/CNK values of 0.98~1.22,are consistent with S-type granite (although features of I-type granite are also present). Andesite porphyrite samples have SiO2 contents of 57.50%~62.15%,K2O content of 2.30%~3.60%,MgO content of 2.27%~4.40%. Both the granodiorite porphyries and andesite porphyrites are enriched in large-ion lithophile element (LILE) and Pb,and depleted in high field strength elements (HFSE) and Ba. The Baerda magmatic rocks have higher εNd(t) values (-7.8^-6.8 ) and lower initial 87Sr/86Sr ratios ( 0.7094~0.7105) relative to melts derived from mature continental crust in the Gangdese. The Baerda magmatic rocks are heterogeneous in zircon εHf(t) values (-4.2^+3.3),yielding younger zircon Hf crustal modal ages of 639(975Ma. Both zircon Hf isotopic data and bulk-rock zircon saturation temperature (768~818℃) indicate that mantle-derived materials may have played an important role in the generation of the Baerda magmatic rocks. We propose that the Baerda magmatic rocks were most likely generated in a setting associated with the slab breakoff of the southward subduction of the Bangong-Nujiang ocean floor,where rising asthenosphere may have provided the heat and material for the melting of lithosphere mantle to generate basaltic magmas. Such basaltic magmas are capable of causing anatexis of upper crust and then mixed with crust-derived melts to form hybrid melts (i.e.,lithosphere mantle-derived basaltic magmas+crust-derived felsic magmas) parent to the Baerda magmatism. We attribute the shift from S-to I-type melts observed in Baerda region to the enhanced contributions of mantle-derived melts during magma mixing.