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

发布时间：2019-06-15 21:27

【摘要】：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。
[Abstract]:In the southwestern section of the Yangtze plate and Huaxia plate junction zone (called the Qin-Hangzhou joint zone), due to the late caprock cover and structure, magmatic intrusion and destruction, the paleo-oceanic crust fragment (ophiolite), which is the symbol of the joint zone, has not been identified so far, which leads to serious differences in the boundary, geotectonic properties and temporal and spatial evolution of the joint zone. Through systematic field geological investigation, it is found that there are a series of meso-basic volcanic rocks along both sides of the Cengxi-Wuzhou fault zone along the northern margin of Yunkai block. The results of geochemistry show that these intermediate-basic volcanic rocks can be divided into three categories with very different properties: (1) the metaphorical medium-basic volcanic rocks exposed in Nuodong Town Camellia oleifera area have slightly deficient large ion lithophile elements and light rare earth elements, while high field strong elements and heavy rare earth elements are slightly enriched, and the negative anomalies of Nb,Ta,P and Ti are not obvious. The zircon LA-MC-ICP-MS U-Pb harmonic age of the plagioclase is 443.7 卤2.2 Ma, which is similar to that of the N-MOB type volcanic rocks. (2) the metamorphosed basic volcanic rocks exposed in Guizhen Dongwei area show slight enrichment of large ion lithophile elements and flat or slightly enriched light rare earth elements, and the weak depletion of Nb-Ta,P and Ti shows their geochemical characteristics similar to those of E-MOB type volcanic rocks, in which the zircon LA-MC-ICP-MS U-Pb harmonic age of plagioclase is 441.3 卤2.4 Ma; (3) on the whole, the medium-basic volcanic (clastic) rocks exposed in Anping town whiteboard and Daxang area are obviously enriched in large ion lithophile elements and light rare earth elements, while the high field strength elements and heavy rare earth elements are relatively deficient, and there are also obvious losses of high field intensity elements such as Nb-Ta,P and Ti, showing similar geochemical characteristics of island arc (IAB) volcanic rocks related to the subduction zone. The zircon LA-MC-ICP-MS U-Pb harmonic age of Anshan crystal clastic tuff is 442.2 卤3.7 Ma. It is shown that there are early Paleo-paleo-oceanic basins in the southwestern section of the Yangtze plate and Huaxia plate junction zone (Qinghang junction zone). The basic volcanic rocks in this area may be formed under the geodynamic background of the continuous subduction of the oceanic basin lithosphere to the Huaxia plate (Yunkai block) during the Caledonian period. The results show that the middle basic volcanic rocks in the Yangtze plate and Huaxia plate (Qinghang junction zone) exist in the southwestern segment of the Yangtze plate and Huaxia plate, and the basic volcanic rocks in this area may be formed in the background of geodynamics.
【作者单位】：桂林理工大学地球科学学院;桂林理工大学广西隐伏金属矿产勘查重点实验室;中国地质科学院矿产资源研究所;
【基金】：广西自然科学基金项目(2013GXNSFAA019272) 中国地质调查局地质调查项目(12120114039501、1212011085408、DD20160124) 桂林理工大学人才引进科研启动基金项目(002401003364) 广西高等学校高水平创新团队及卓越学者计划项目(桂教人〔2016〕38号) 桂林市漓江学者基金项目(2013-5)联合资助
【分类号】：P588.14

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