南羌塘增生过程的中-晚三叠世岩浆记录:藏北玛依岗日-角木日地区基性岩墙

Middle-Late Triassic magmatic records for the accretionary processes of South Qiangtang accretionary terrane: The mafic dykes in Mayigangri-Jiaomuri area,North Tibet

古特提斯洋向北俯冲形成增生杂岩,它们向南增生构成了南羌塘增生地体,且增生期内发育的岩浆岩可以用来研究南羌塘的增生过程。在南羌塘增生杂岩带内,由北向南,发育有玛依岗日(MG)辉长岩墙、角木日(JM)辉长辉绿岩墙。锆石U-Pb测年结果显示,两者分别形成于237. 1±2. 3Ma和230. 7±1. 8Ma,为南羌塘增生期内岩浆岩。它们的岩石地球化学特征均介于OIB与E-MORB之间,富集Ti、Nb,以及LREE和LILE。Nd-Pb同位素结果显示两者均起源于富集地幔源区。Sm/YbLa/Yb图解显示,两者均起源于尖晶石-石榴石二辉橄榄岩源区,相对于JM辉长辉绿岩MG辉长岩部分熔融程度较高。两者Mg#和Ni、Cr特征表明,MG辉长岩分异程度高于JM辉长辉绿岩。Th/Nb-La/Nb图解显示,相对于JM辉长辉绿岩MG辉长岩经历了一定程度的地壳混染。综合研究认为,MG辉长岩和JM辉长辉绿岩为古特提斯洋壳俯冲过程中软流圈上涌的两次岩浆活动的产物,并受俯冲洋壳和地幔楔影响。MG辉长岩和JM辉长辉绿岩记录了南羌塘的增生过程,它们为相关研究提供了岩浆岩证据。

The northward subduction of Paleo-Tethys Ocean (PTO) formed the accretionary complex, which constructed the South Qiangtang accretionary terrane (SQAT). Therefore, the magmatic rocks formed during the accretionary period can be used to study the accretionary processes of SQAT. From north to south, the Mayigangri (MG) gabbro and the Jiaomuri (JM) gabbro-diabase are developed and outcropped in the accretionary complex belt of SQAT. Zircon SHRIMP U-Pb dating results show that the MG gabbro and the JM gabbro-diabase formed at 237.1±2.3Ma and 230.7±1.8Ma respectively, developed in the accretionary period of SQAT. Their characteristics of whole rock geochemical data are between those of OIB and E-MORB. Both of rocks contain high Ti and Nb contents, and are enriched in LREE and LILE. The Nd-Pb isotopic data indicate that both of them were derived from enriched mantle. Furthermore, the Sm/Yb-La/Yb diagram shows that both of them were derived from spinel-garnet lherzolite, while the MG gabbro magma was generated by higher degree of partial melting of source mantle than the JM gabbro-diabase. In addition, their Mg# as well as the contents of Cr and Ni suggested that the MG gabbro experienced higher degree of fractionation than the JM gabbro-diabase. On the other hand, the Th/Nb-La/Nb diagram shows that the MG gabbro underwent a certain degree of crustal contamination compared with the JM gabbro-diabase. Comprehensively, both of the MG gabbro and the JM gabbro-diabase were the products of twice magmatic events caused by the upwelling of asthenosphere during the oceanic crust subduction of PTO. Meanwhile, both of them were influenced by the subduction slab and mantle wedge. Consequently, the MG gabbro and the JM gabbro-diabase recorded the accretionary processes of SQAT, and thus provide magmatic evidence for relevant research.