昆仑河早古生代两期埃达克质侵入岩的发现及其对东昆仑碰撞造山过程的启示

Discovery of two stages of the Early Paleozoic adakitic intrusive rocks in the Kunlun River area, East Kunlun: Implications for collisional orogenic processes

早古生代志留纪-泥盆纪是东昆仑原特提斯洋闭合和碰撞造山过程发生的重要时期。在该阶段出现有榴辉岩和大量A型花岗岩、镁铁-超镁铁质岩,并产出有夏日哈木镍矿、白干湖钨锡矿等一批重要的矿产资源。然而,东昆仑早古生代详细碰撞造山的深部过程、榴辉岩的折返机制等问题还没有得到合理的解释。在东昆仑造山带昆仑河地区新发现的早古生代埃达克质侵入岩可能为上述问题的解决提供了重要依据。LA-ICP-MS锆石U-Pb测年结果表明昆仑河地区存在晚奥陶世(446Ma)和晚志留世(427-425Ma)两期埃达克质侵入岩。晚奥陶世埃达克质侵入岩为花岗闪长岩,SiO2(67.55%-68.21%)和Al2O3(14.59%-15.89%)含量较高,富Na2O(4.91%-5.15%)、贫K2O(1.54%-1.64%),亏损重稀土,Y含量是7.76×10^(-6)-8.61×10^(-6),Yb含量是0.67×10^(-6)-0.93×10^(-6),高Sr(484×10^(-6)-621×10^(-6))和Sr/Y比值(56-80),Cr(31.2×10^(-6)-38.3×10^(-6))和Ni(19.0×10^(-6)-22.5×10^(-6))含量较高,具有亏损的全岩Nd同位素组成,εNd(t)为+1.1-+1.2,其岩浆成因锆石的εHf(t)分成两组:主要岩浆锆石为+2.2-+10.6,另外一组岩浆锆石为-11.5--1.3。晚志留世埃达克质侵入岩主要为花岗闪长岩、花岗岩,高SiO2(66.36%-71.14%)和Al2O3(15.5%-19.65%),富Na2O(5.65%-7.68%),贫K2O(0.68%-1.47%),强烈亏损重稀土,Y为1.90×10^(-6)-5.02×10^(-6),Yb为0.16×10^(-6)-0.51×10^(-6),高Sr(362×10^(-6)-1100×10^(-6))和Sr/Y比值(130-514),具有弱富集的全岩Nd同位素组成,εNd(t)为-1.5--1.2,岩浆锆石εHf(t)主要分布在-10.2-+9.3之间。本文认为晚奥陶世埃达克质侵入岩最可能是俯冲洋壳熔融形成的埃达克质岩浆在上升过程中与同期古老大陆地壳来源岩浆发生了混合后形成的,而晚志留世埃达克质侵入岩可能是加厚下地壳部分熔融形成。结合区域地质资料,本研究认为东昆仑在晚奥陶世(446Ma)还存在大洋板片的俯冲和熔融,约在436-425Ma发生了大陆碰撞、板片断离和高压-超高压(HP-UHP)变质岩折返,并诱发了埃达克质、镁铁-超镁铁质和A型花岗质岩浆的产生。

The Early Paleozoic(Silurian to Devonian interval) is the main stage for the closure of the Proto-Tethys Ocean and collisional orogenesis in the East Kunlun. Abundant rocks, including eclogites, A-type granites, mafic-ultramafic rocks, as well as the associated mineral deposits, such as the Xiarihamu Ni deposit and Baiganhu W-Sn deposit, were formed during this period. However, the detail collisional orogenic process and exhumation mechanism of eclogites in the Early Paleozoic remain unclear. The discovery of the Early Paleozoic adakitic intrusive rocks in the Kunlun River area may provide the important evidence for the solution to these questions. LA-ICP-MS zircon U-Pb dating indicates that there are Late Ordovician(446 Ma) and Late Silurian(427-425 Ma) adakitic intrusive rocks in the Kunlun River area. The Late Ordovician adakitic rocks are granodiorites and exhibit high SiO2(67.55%-68.21%), Al2O3(14.59%-15.89%), and Na2O(4.91%-5.15%), and low K2O(1.54%-1.64%). They are depleted in heavy rare earth elements with low Y(7.76×10^(-6)-8.61×10^(-6)) and Yb(0.67×10^(-6)-0.93×10^(-6)) and high Sr(484×10^(-6)-621×10^(-6)), Sr/Y ratios(56-80), Cr(31.2×10^(-6)-38.3×10^(-6)), and Ni(19.0×10^(-6)-22.5×10^(-6)). They have relatively depleted whole-rock Nd isotopic compositions with positive εNd(t) values(+1.1-+1.2). The magmatic zircons of the Late Ordovician(446 Ma) adakitic granodiorites have two groups of εHf(t) values: +2.2-+10.6 for most of magmatic zircons and-11.5--1.3 for some of them. The Late Silurian adakitic intrusive rocks consist of granodiorites and granites. They contain high SiO2(66.36%-71.14%), Al2O3(15.5%-19.65%) and Na2O(5.65%-7.68%), but low K2O(0.68%-1.47%). They are strongly depleted in heavy rare earth elements with low Y(1.90×10^(-6)-5.02×10^(-6)), Yb content(0.16×10^(-6)-0.51×10^(-6)), high Sr(362×10^(-6)-1100×10^(-6)), and Sr/Y ratios(130-514). They have relatively enriched whole-rock Nd isotopic compositions with negative εNd(t) values(-1.45--1.24). The magmatic zircons from Late Silurian(427-425 Ma) adakitic intrusive rocks have variable εHf(t) values ranging from-10.2 to +9.3. We suggest that the Late Ordovician adakitic intrusive rocks were most likely formed by the mixing between coeval subducted ocean crust and ancient continental crust-derived magams during ascent, whereas the Late Silurian adakitic intrusive rocks were generated by partial melting of the thickened lower crust. Taking into account regional geological data, we suggest that, in the East Kunlun, the subduction and partial melting of oceanic crust happened during Late Ordovician(446 Ma), and several other processes including the continental collision, slab break-off, exhumation of the high pressure-ultra high pressure(HP-UHP) metamorphic rocks occurred simultaneously during 436-425 Ma, which triggered the generation of adakitic, mafic-ultramafic, and A-type granitic magmas.