华北克拉通北部地幔交代作用与早二叠世超镁铁质岩成因

Mantle metasomatism and generation of Early Permian ultramafic rocks in northern North China Craton

华北克拉通北部高寺台晚古生代环状杂岩体是一套富单斜辉石的超镁铁质岩,是研究华北北部地幔碳酸盐化熔体交代作用的理想对象,同时也是华北克拉通北部晚古生代岩浆活动与深部碳循环联系的纽带。作者在岩体西部边缘识别出一套早二叠世早期富铁角闪单斜辉石岩,形成时代为297.8±3.4Ma。结合岩体东部辉石岩早泥盆世和早石炭世的成岩年龄,表明高寺台超镁铁质杂岩体可能是晚古生代多期岩浆侵位的产物。其次,高寺台环状超镁铁质杂岩体缺少大面积辉长岩和闪长岩、硫化物和Cu-Ni-PGE矿化,单斜辉石成分具有裂谷带堆晶岩矿物特征,角闪石也具有相对低的结晶温度(933~947℃)和压力(287~323MPa)及水含量(4.06%~5.10%),而且区域上尚未发现同期环状杂岩体和蛇绿岩,与岛弧拼贴到陆块后沿俯冲带延伸数万米侵位的阿拉斯加型环状杂岩体明显不同。另外,前人的研究显示,与铬铁矿中富镁单斜辉石包裹体平衡的苦橄质母岩浆显示出OIB的特征,具有强烈的重稀土分馏、富Nb和高Nb/La,异常高的Ca/Al和(La/Yb)N,低的Ti/Eu、Zr/Sm和Hf/Sm,这些特征都表明苦橄质母岩浆起源于石榴石相软流圈地幔,且源区遭受过显著的碳酸盐熔体交代。单斜辉石岩中单斜辉石及角闪石成分及其平衡岩浆成分记录了母岩浆与岩石圈地幔相互作用后的岩浆演化过程,也显示出比母岩浆弱的源区碳酸盐熔体交代印记。此外,岩浆房在地壳浅部经历了多期深部岩浆补给和富铁熔体贯入,同时带来热量和挥发分,触发晶粥体活化,促进了岩浆的上升运移。高寺台环状超镁铁质杂岩体具有板内成因特征,可能是陆内岩石圈伸展的产物。华北克拉通北部晚石炭世-早二叠世三个岩浆事件峰期很好地对应着全球三次升温和CO_(2)回升事件,岩石圈伸展能有效促进强烈的中酸性岩浆活动及起源于碳酸盐化地幔的基性岩浆释放出大量的CO_(2),它们可能是晚石炭末期-早二叠世早期全球大气CO_(2)回升的贡献者之一。

The Gaositai Late Paleozoic near-concentric clinopyroxene-rich ultramafic complex in northern Hebei is an ideal object to revealing carbonate metasomatism in mantle beneath the northern North China Craton(NCC).It might establish a connection between the Late Paleozoic magmatic activity and deep carbon cycle in northern NCC.We have recently identified an Early Permian iron-rich amphibole clinopyroxenite from the western margin of the ultramafic pluton,with a formation age of 297.8±3.4Ma.Combined with the intrusive ages of Early Devonian and Early Carboniferous pyroxenites in the eastern part of the pluton,it is suggested that the Gaositai ultramafic complex may have been produced by multi-stage Late Paleozoic magmatic intrusion.Furthermore,the Gaositai near-concentric ultramafic complex lacks voluminous gabbro and diorite,sulfide and Cu-Ni-PGE mineralization.The clinopyroxene also shows rift-related origin,and the amphiboles in it have relatively low crystallization temperatures(933~947℃),pressures(287~323MPa)and water contents(4.06%~5.10%).No coeval near-concentric ultramafic complexes and ophiolites have been found in the northern NCC.These observations are remarkably different from the Alaskan-type near-concentric complex,which extended several hundred kilometers along the subduction zone after the accretion of island arc to the continental block.In addition,previous studies have shown that the magnesium-rich clinopyroxene inclusions in chromites as well as their equilibrium picritic parent magma exhibit the OIB signatures,i.e.,strong heavy rare earth element fractionation,rich in Nb and high Nb/La ratios,extremely high Ca/Al and(La/Yb)N ratios,but low Ti/Eu,Zr/Sm and Hf/Sm values.The picritic parent magma originated from the garnet-facies asthenosphere mantle,which had undergone significant carbonate metasomatism.The compositions of clinopyroxenes and amphiboles and their equilibrium magma in the Early Permian clinopyroxenite recorded the magma evolution process following the interaction between the parent magma and the lithospheric mantle,and revealed a weaker imprint of carbonate metasomatism than that of the parent magma.Furthermore,the magma chamber experienced multiple stages of deep magma recharge and iron-rich melt penetration in the shallow crust level,which supplied both heat and volatiles,activated the crystal mush,and drove magma upward and migration.The Gaositai near-concentric ultramafic complex has an intraplate origin,possibly as a result of intracontinental lithosphere extension.On the other hand,the three peaks of the Late Carboniferous to Early Permian magmatic events in the northern NCC closely match the three episodes of global warming and CO_(2) recovery.Strong intermediate-felsic magmatic activity and basaltic magma originated from the carbonated mantle could be promoted effectively by lithosphere extension to release huge amounts of CO_(2).They might have been one of the contributors to the global atmospheric CO_(2) recovery during the Late Carboniferous to Early Permian.