扬子陆块北缘花山群:弧后盆地火山-沉积岩系而非蛇绿混杂岩

Huashan Group in Northern Margin of Yangtze Block: A Suite of Back-Arc-Basin Volcanic-Sedimentary Strata but Not Ophiolite Mélange

扬子陆块北缘花山群曾被作为南秦岭古生代勉-略蛇绿岩的东延部分,后又被厘定为新元古代"俯冲增生杂岩",至今争议甚多.为明确花山群的大地构造含义,了解扬子陆块参与新元古代Rodinia超大陆的演化过程,采用碎屑物源分析、沉积物氧化/还原指标、火成岩构造环境判别、锆石U-Pb定年等方法研究了花山群六房咀组的物质属性与时代.六房咀组的沉积盆地属于氧化条件的边缘海,其中沉积岩的碎屑物包含扬子大陆的稳定砂屑和玄武玻璃质岩屑两组来源,暗示该组火山岩与沉积岩为同期伴生的产物,而非后期构造楔入的岩块.该组玄武岩兼具洋中脊和岛弧的双重地球化学特征,属于弧后盆地玄武岩(BABB).以锆石U-Pb法测得玄武岩的形成年龄为823.9±5.5 Ma和829.9±6.9 Ma,并获得1 637 Ma、2 670 Ma两组捕获锆石年龄,表明火山通道下部存在古老大陆基底,并非洋中脊环境.花山群六房咀组总体为一套新元古代弧后盆地沉积-火山岩建造,而非古生代或新元古代蛇绿岩混杂岩.新元古代晚期,扬子陆块北缘发生了板块汇聚和俯冲作用,相对于同期的Rodinia超大陆裂解事件存在一定的滞后.

The Huashan Group, which was once regarded as the eastward extension part of the Paleozoic"Mianlüe ophiolite"or the Neoproterozoic subduction-accretion-complex, has been controversial so far. To identify its tectonic significance and its relationship with the Rodinia supercontinet, the petrogenesis and age of the Liufangzui Formation by clastic tracing, redox index,tectonic discrimination and zircon U-Pb dating were studied. The sedimentary basin of the Liufangzui Formation is a marginal sea with oxidation condition, in which the clastic rocks had two sources, including sand debris from the Yangtze Block and basaltic glass debris, suggesting that the strata and the basalt were associated products in the same period, but not emplacements in a melange. The basalts which have dual-characteristics between MORB and IAB, are actually BABB. 823.9±5.5 Ma and 829.9±6.9 Ma of zircon ages were obtained, as well as 1 637 Ma and 2 670 Ma of xenocrystal zircon ages from these basalts, which implies that there was probably an continental basement in the lower part of the volcanic channel, but not a mid-ocean-ridge. The Liufangzui Formation of the Huashan Group is a Neoproterozoic back-arc-basin sedimentary-volcanic strata rather than an ophiolitic mélange. In Late Neoproterozoic, a plate-subduction movement took place in the northern margin of Yangtze Block,which lagged behind the global Rodinia supercontinent breakup event.