云开地块北缘加里东期中-基性火山岩的厘定:钦-杭结合带南西段早古生代古洋盆存在的证据

在扬子板块和华夏板块结合带(称之为钦-杭结合带)南西段,由于后期盖层覆盖和构造、岩浆侵入破坏,作为结合带标志的古洋壳残片(蛇绿岩)至今未被识别出来,从而导致了前人对该结合带的边界、大地构造性质及其时空演化等问题存在严重分歧。笔者通过系统的野外地质调查发现,在云开地块北缘的岑溪地区,沿着岑溪-梧州断裂带两侧出露有一系列呈岩片状产出的(变质)中-基性火山岩块。地球化学的研究结果表明,这些(变质)中-基性火山岩可分为性质截然不同的三大类:(1)糯垌镇油茶林场一带出露的变质中基性火山岩具有略亏损的大离子亲石元素和轻稀土元素,而高场强元素和重稀土元素轻微富集,且Nb、Ta、P和Ti的负异常不明显,显示出与N-MORB型火山岩相似的地球化学特征,获得其中斜长阳起石岩的锆石LA-MC-ICP-MS U-Pb谐和年龄为443.7±2.2Ma;(2)归义镇洞尾一带出露的变质基性火山岩则表现出轻微富集大离子亲石元素和具有平坦或略富集的轻稀土元素,并出现Nb-Ta、P和Ti的微弱亏损现象,显示出其与E-MORB型火山岩相似的地球化学特征,获得其中斜长角闪岩的锆石LA-MC-ICP-MS U-Pb谐和年龄为441.3±2.4Ma;(3)安平镇白板、大爽一带出露的中-基性火山(碎屑)岩总体上表现为明显富集大离子亲石元素和轻稀土元素,而高场强元素和重稀土元素相对亏损,其间还出现Nb-Ta、P和Ti等高场强元素的明显亏损,显示出与俯冲带有关的岛弧型(IAB)火山岩相似的地球化学特征,获得其中安山质晶屑凝灰岩的锆石LA-MC-ICP-MS U-Pb谐和年龄为442.2±3.7Ma。由此表明,扬子板块和华夏板块结合带(钦-杭结合带)南西段有早古生代古洋盆的存在,本区N-MORB、E-MORB和IAB型中基性火山岩可能是在加里东时期该洋盆岩石圈持续向南东俯冲消减至华夏板块(云开地块)之下的地球动力学背景下形成的产物。

The confirmation of Caledonian intermediate-mafic volcanic rocks in northern margin of Yunkai block: Evidence for Early Paleozoic paleo-ocean basin in southwestern segment of Qinzhou-Hangzhou joint belt

Because the later sedimentary cover and destruction of structure and magma intrusion, the residual paleo-oceanic crusts (ophiolite) were regarded as one of the most important indicators of joint belt that has not been identified in southwestern segment of joint belt between Yangtze and Cathaysia plates (so-called Qinzhou-Hangzhou joint belt), which led to serious differences in the boundary of joint belt, tectonic nature and temporal-spatial evolution and so on. Based on systematic field geological survey reveals that there outcrop a series of (metamorphic) intermediate-mafic volcanic rock blocks along both sides of the Cenxi-Wuzhou fault in Cenxi area, the northern margin of Yunkai block. Geochemical results suggest that these (metamorphic) intermediate-mafic volcanic rocks can be divided into three distinct types:(1) metamorphic intermediate-mafic volcanic rocks from the Camellia tree farm in Nuodong town, which are characterized by slightly depleted in large-ion incompatible elements (LILEs) and LREEs and slightly enrichment in high field strength elements and HREEs with no obvious Nb, Ta, P and Ti negative anomaly. These geochemical features show that these metamorphic intermediate-mafic volcanic rocks are N-MORB type tholeiites typical of those formed in mid-ocean ridge tectonic settings and a plagioclase actinolite rock sample yielded a zircon LA-MC-ICP-MS U-Pb concordia age of 443.7±2.2Ma; (2) metamorphic mafic volcanic rocks from the Dongwei region in Guiyi town, characterized by slightly enrichment in LILEs and flat-slightly enrichment in LREEs with only slight Nb-Ta, P and Ti depletions, which are analogous to those of E-MORB type tholeiites formed in back-arc basin tectonic settings and a plagioclase amphibolite sample yielded a zircon LA-MC-ICP-MS U-Pb concordia age of 441.3±2.4Ma; (3) intermediate-mafic volcanic (clastic) rocks from the Baiban-Dashuang region in Anping town, which are characterized by pronounced enrichment in LILEs and LREEs and relatively depletion in HFSEs and HREEs with pronounced Nb-Ta, P and Ti depletion in them. Such signatures display geochemical characteristics of typical subduction-related arc volcanic rocks and an andesitic crystal tuff sample yielded a zircon LA-MC-ICP-MS U-Pb concordia age of 442.2±3.7Ma. These results imply that there is an Early Paleozoic paleo-ocean basin in southwestern segment of joint belt between Yangtze and Cathaysia plates (Qinzhou-Hangzhou joint belt), and N-MORB, E-MORB and IAB type intermediate-mafic volcanic rocks in the region were most likely generated in a setting associated with this oceanic lithosphere continued southeastward subduction under the Cathaysian (Yunkai) Block during the Caledonian period.