中扬子区成岩成矿板块构造模型的初步探讨

前言本区分布有广泛的花岗岩类及铁、铜、稀有金属矿床和矿化,是我国重要的铁、铜矿产区域之一,大冶式铁矿就产于此。多年来,不同的作者将本区铁铜矿床归之为接触交代成矿和热液成矿、火山同生沉积成矿、矿浆成矿、层控成矿、板块成矿。作者根据近些年来各单位在这一地区所获得的岩石学、地球化学及同位素地质学资料的综合研究,认为这些花岗岩类及与其有关的金属矿床和矿化可能是在燕山旋回时,扬子准地台板块与大别板块(古板块)相撞,大别

A Tentative Plate Model For The Rock-Forming And Ore-Forming Processes In The Middle Yangtze Valley

The petrological, geochenucal and isotopic data obtained in recent years suggest that the formation of granitoids and their related ore deposits might be ascribed to the upward movement of the partly molten oceanic crust of ophiolite suite subducting and sinking along the paleo-Benioff zone when the Yangtze paraplatform plate collided with the Dabei paleoplate during the Yenshanian movement. The rock-forming and ore-forming materials were derived mainly from this oceanic crust and partly from the country rocks assimilated by the ascending regenerated magma. The major evidence for this plate tectonic model is as follows: 1. The depth of the Benioff zone dominates two distinct types of magmatic rocks and their ore deposits in Daye and Mufushan regions. K₂O and K₂O/Na₂O in magma increase with the depth and the distance from the Benioff zone. ln areas approaching the Benioff zone or at shallow depths (Daye area) diorite- granodiorite- granite of zircon-apatite-sphene-magnetite type and related volcanic rock sequence occur, characterized by Na2O>K2O, containing abundant chalcophile and siderophile elements, and associated with such deposits as Fe, Fe-Cu, Cu, S(pyrite), Cu-Mo, Cu-W, W-Mo, Pb-Zn (accompanied by certain amounts of Au and Ag) while in areas distant from the Benioff zone or at deep depths (Mufushan area), biotite monzonitic granite-muscovite-biotite granite and granopegmatite of zircon-xenotime-monazite-ilmenite type are found, showing the character of K₂O>NaO, having enriched lithophile elements, related to rare element deposits such as monazite, beryl and Nb-Ta and also to Pb-Zn deposits of quartz-barite-fluorite vein type. 2. The Cr, Ni, Co contents of granites at Daye and Cr content of granites at Mufushan are unexceptionally higher than the average abundances of these elements in the same sort of rocks, with Cr content even reaching 500 ppm somewhere; Fe and Cu ores generally contain 0~0.032 g/t Pt, 0.01~0.015 g/t Pd, the highest content being 0.2 g/t; Pt+Pd content of single chalcopyrite, pyrite and magnetite minerals can reach as high as 0.94 g/t. In addition, drainage sediments from intrusives of Daye area contain approximately the same amounts of B as those from oceanic crust of the Dabieshan ophiolite suite (15-40 ppm). 3. Rb-Sr and K-Ar ages of granites and ore deposits are ll0-165m. y., indicating the Yenshanian cycle. Old materials of pre-Sinian period, however, are also present, as evidenced by Pb²⁰⁷/Pb²⁰⁴ age of zircon from the Tieshan intrusive mass (315m.y.) and by Pb²⁰⁷/Pb²⁰⁴ versus Pb²⁰⁶/Pb²⁰⁴ age of galena from the Cu-Pb-Zn deposit (2200m. y.). These two ages are principally in agreement with the age of the Dabieshan ophiolite suite. 4. Paleotransform fault is believed to have played some substantial role in governing the migration of magma and ore-forming fluids during the activity of Benioff zone.