摘要
松卡岩体地处藏南冈底斯岩基南缘的东段,主要由闪长岩和花岗岩脉组成,为详细研究与大洋俯冲有关的岩浆过程提供了良好的记录。锆石U-Pb地质年代学研究表明松卡岩体闪长岩的形成年龄为97Ma,具有高Sr、低Y和高Sr/Y比的特点,显示出埃达克质岩石的特征,同时具有高MgO含量和高Mg~#值,以及较低的~(87)Sr/~(86)Sr(t)(~0.7042)和较高的ε_(Nd)(t)(+4.6~+8.8),是俯冲新特提斯洋壳和少量海洋沉积物部分熔融并随后与上覆地幔楔发生相互作用形成的。与91Ma松卡埃达克质岩石相比,97Ma松卡埃达克质闪长岩具有更多地幔物质参与。松卡花岗岩脉的锆石U-Pb年龄为93Ma,同样具有较低的~(87)Sr/~(86)Sr(t)(0.7046)值和较高的ε_(Nd)(t)(+7.5~+7.8)值,可能是具有亏损地幔特征的中基性岩浆经过分离结晶作用演化而来。上述数据表明,在100~90Ma期间,较年轻的俯冲新特提斯板片可能发生过部分熔融,板片部分熔融熔体与上覆地幔楔的相互作用是形成高镁闪长岩(安山岩)的重要机制之一。
The Songka pluton is located in the eastern part of the southern margin of Gangdese batholith,southern Tibet. It is mainly composed of diorite with minor dikes of granitic compositions. Zircon U-Pb analyses shows coarse grained and fine grained diorite formed at 97 Ma,whereas the granitic dike at 93 Ma,respectively. The Songka diorites are characterized by:( 1) high Sr,but low Y contents and high Sr/Y ratios;( 2) high MgO and Mg~#; and( 3) low ~(87) Sr/~(86)Sr( t)( ~ 0. 7042),but high ε_(Nd)( t)( + 4. 6 ~ +8. 8) values. Most of the Songka diorites,especially those coarse grained diorite,display geochemical features resembling those of high-Mg adakitic diorites. Such features could be explained by interactions of melts derived from subducting Neo-Tethyan slab( oceanic crust plus minor amount of marine sediments) with the overlying mantle wedge. Compared with the 91 Ma Songka adakitic rocks,formation of the 97 Ma Songka adakitic diorite requires a higher degree of involvement of mantle materials. The slightly younger granitic dikes are also characterized by low ~(87) Sr/~(86)Sr( t)( ~ 0. 7046) but high ε_(Nd)( t)( + 7. 5 ~ + 7. 8) values,which suggests that they represent evolved melts from more mafic magmas originated from the depleted mantle. Data presented in this study indicate that the relatively young subducting Neo-Tethyan slab could experience partial melting during the time interval of 100 ~ 90 Ma,and the interaction of slab-derived melts with the overlying mantle wedge is a key process to produce high-Mg diorite.
引文
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