松潘-甘孜地块丹巴二叠纪玄武岩的主、微量元素和Sr-Nd同位素研究:岩石成因与构造意义

松潘-甘孜地块的丹巴二叠纪玄武岩(大石包组)具有较高的TiO2含量(>2%)和高的Ti/Y比值(平均519),显示LREE富集、HREE亏损的右倾型稀土配分型式((La/Yb)N=4.2~13.6),εNd(t)=-0.33~2.70,具有洋岛玄武岩(OIB)地球化学特征,形成于大陆板内环境。其源区来自原始地幔始于石榴子石稳定区的低程度部分熔融,岩浆上升过程中有来自地壳物质的加入,因而其不相容元素比值如Zr/Nb(4.41~13.09)、La/Nb(1.03~1.80)和Th/La(0.08~0.18)等,以及初始的87Sr/86Sr比值(0.706008~0.707257)均表现出不同程度的富集特征,岩浆演化早期经历了以辉石、橄榄石为主的分离结晶作用。该套玄武岩的元素-同位素地球化学特征和源区性质类似于峨眉山溢流玄武岩的高钛(HT)系列,因此认为其是峨眉山地幔柱活动的产物,属于峨眉山大火成岩省(ELIP)的一部分。松潘-甘孜地块和扬子西缘晚古生代以前地层的可比性以及峨眉山溢流玄武岩的分布特征显示,松潘-甘孜洋盆伴随着扬子克拉通的裂解而打开,并且可能都与峨眉山地幔柱有关,是地幔柱活动的浅部地质响应。

GEOCHEMICAL AND SR-ND ISOTOPIC STUDY ON PERMIAN BASALTS FROM DANBA AREA,SONGPAN-GANZE TERRANE:PETROGENESIS AND TECTONIC IMPLICATION

The Permian marine basalts at the Danba, Songpan-Ganze terrane,are located to the west of the Longmenshan fault in northwestern Sichuan Province. These basalts are characterized by high-Ti tholeiites,which have high TiO2 contents (>2%) and Ti/Y ratios (~519). And they have OIB-type incompatible elemental ratios and REE and spidergram patterns. Moreover, their εNd(t) values range from -0.33 to 2.70. These characteristics in combination with enriched incompatible elemental ratios like Zr/Nb(4.41~13.09), La/Nb(1.03~1.80) and Th/La(0.08~0.18), as well as initial 87Sr/86Sr(0.706008~0.707257), indicate that there were contribution from crustal materials during the ascent of magma. The overall characteristics of the Danba Permian basalts are comparable with those of the Emeishan high-Ti basalts, suggesting that they were generated within continental plate and were resulted from the Emeishan mantle plume. Based on the petrographies, major and trace elemental and isotopic geochemistry, the Danba Permian basalts are virtually part of the Emeishan large igneous province (ELIP), which were formed by mantle plume activity at ~260Ma. These suggest that ELIP occurs beyond the Yangtze Craton and extends to Songpan-Ganze terrane. Furthermore, the temporal and spatial coincidence between ELIP basalt eruption and rifting of the Yangtze Craton, together with kilometer-scale regional doming prior to eruption, implys that continental breakup might be a shallow response to the Emeishan plume.