The origin of the early Proterozoic Kuandian Complex: evidence from geochemistry

Abstract

The Kuandian Complex is a rarely preserved Early Proterozoic volcanic suite, formed 2. 3 - 2. 4 Ga ago. It is located in an Early Perterozoic mobile belt which is bounded by the Archean Rangrim and Ryonggang Blocks of the northeastern Sino-Korea Craton. The Complex is mainly made up of amphibolite, a variety of gneisses, and layered granite. Petrological and geochemical studies show that the protoliths of the complex are associations of bimodal volcanics and anoro-genic granites. The Kuandian amphibolites are depleted in Nb, Ta, P and Ti, and enriched in LILE, e. g. K, Rb and Cs, with pronounced depletion of Sr relative to Nd and Pb; La/Nb ratios are higher than 1 (1. 75 to 5. 18). The trace element patterns of the amphibolites are similar to continental flood basalts formed by Gondwana break-up, such as those in South Karoo and Tasmania, which shows continental contamination, evalues ranging from 0. 70 to 1. 94 of the Kuandian amphibolites and the relationships between Nb/Yb and La/Yb suggest that contamination of basaltic magma happened in the mantle, rather than along the conduit. Isotope ratios of 208Pb/204Pb,207Pb/204Pb,206Pb/204Pb,143Nd/I44Nd and 87Sr/86Sr indicate that the magma was derived from a contaminated mantle source like DMM or a mixture of DMM and EM2. The Kuandian complex has Dupal anomaly, as is the case with some continental basalts in the south hemisphere , e. g. in South Karoo and Tasmania. Petrochemical modeling proposes that the Kuandian gneiss, granite, and amphibolite came from the same parental magma, being products of strong fractional crystallization. Protoliths of the Kuandian complex were formed in an extensional tectonic setting during the transition from continental crust to oceanic crust.The formation of the Kuandian complex indicates that 2. 3 to 2. 4 Ga ago, the tectonic evolution of the Sino-Korea Craton was different from that of other well-studied Precambrian era-tons, e. g. the North America shield, European platform and Australian continent in that strong volcanic eruption resulted in its accretion. Besides, the contaminated magma source with a Dupal anomaly for the complex indicates that crustal-mantle convection whose scale was similar to that of the present plate tectonics had occurred at least before the formation of the Kuandian complex (2.3-2.4 Ga B. P. ).

自 Armstrong[1,2] 提出地壳再循环模式以来, 越来越多的地质和地球化学证据表明组成现今大陆的绝大部分岩石是在地球早期形成的。Condie 根据结晶岩U -Pb 年龄数据并结合钦同位素提出了地壳形成的两个主要生长期为2, 6 – 2.8Ga 和1.7 – 1.9Ga [3]。后来在总结元古宙构造演化时, Condie 注意到在北美大陆缺失2.0 – 2.4Ga 这段期间的岩石年龄记录, 因而提出了有可能存在2.0 - 2.4Ga 的地壳生长间断期(crustal generation gap)[4] 。近些年来随着同位素地质年代学研究的深入, 已有零星资料报道在西非[5,6] 和中国[7-9] 等地出露有这段时期形成的岩石。无疑有关这段时期岩石成因的研究, 对于了解早元古代地壳生长过程具有重要意义。 宽甸杂岩形成于2.3 – 2.4Ga[9], 是目前我国较少保留的早元古代变质岩系之一。长期以来有关这套岩系的成因及其形成时的大地构造环境一直存在着争议。张秋生等较早提出辽吉岩套时曾认为这套岩系是形成于大陆裂开优地槽相的一套同源岩浆演化系列[10,11] 。姜春潮等认为是形成于洋壳的一套海底火山喷发沉积岩系[12] 。白瑾等则提出这套变质岩系是板块聚敛(岛弧)作用的产物[13] 。本文根据这套变质岩系的主要元素、微量元素和同位素地球化学特征,重点讨论这套火成岩系的岩石学成因过程包括其可能的岩浆来源、成因演化及其形成的构造背景, 并进而探讨宽甸杂岩在早前寒武纪中朝克拉通演化过程中的地质意义。