论扬子地台与华北地台属同一个岩石圈板块

    THE YANGTZE PLATFORM AND THE NORTH CHINA PLATFORM AS OF THE SAME LITHOSPHERIC PLATE

    • 摘要: 本文运用“区域控制论”和“成矿作用不可逆论”理论,首次从6个方面论证了扬子地台与华北地台是同一个岩石圈板块。并根据板块“基底”深度和板块边界切割深度的不同将其分为三类;地壳(小)板块,莫霍面亚板块和岩石圈板块,华北地台与扬子地台在各地质发展阶段中多数属于莫霍面亚板块,少数阶段为地壳小板块。文中详细论述了秦岭构造带是一个复杂的多旋回裂开—碰撞的重叠造山带,它经历过5个演化阶段和9个演化期,其中包括6次裂开(裂解)和6次挤压、碰撞,以及陆壳塑性推覆和刚性块断作用的复杂演化过程。研究证明,秦岭构造带虽然张裂次数多,但是历次张裂深度都不大,一般多为几十公里;虽然岩浆活动很频繁,但是超镁铁岩不发育,尤其纯橄岩类较少见,因此本区蛇绿岩套都不标准、不典型,作者称为”类蛇绿岩套”,代表了过渡型洋壳的地质环境,说明秦岭构造带多旋回的裂开—碰撞演化过程都是在同一个岩石圈板块内进行的。确定秦岭构造带地壳类型以过渡壳为主,因此其金属成矿作用以贵金属Au、Ag和有色金属Pb、Zn、Mo、Cu等为主,尤其是Au为整个演化过程中的贯通元素,具有巨大的经济意义。本文提出,晚古生代时期扬子地台与华北地台是整体从赤道附近向北漂移的,整体漂移过程就是扬子地台最后一次向华北地台挤压、并使南秦岭过渡壳海槽逐渐收缩、直至中三叠世碰撞和完全封闭的过程。由于扬子地台和华北地台始终属于同一个岩石圈板块,因此它们既不属于世界南方的冈瓦那古陆,也不属于世界北方的劳亚古陆,而是属于可能在世界南,北方之间的第三个古大陆(可能是“古太平洋古陆”)的一部分。

       

      Abstract: By using the theories of regional control and irreversihility in mineralization, the authors suggest that the Yangtze and North China platforms have been of one and the same lithospherie plate. This conclusion is supported by a comparison between the two platforms in six aspects, i.e. metallogenic formations of iron and gold,geochronology,magmatism and magmatie rocks,formation age of khondalite and geophysical fields of their basement. The first breakup in the Qinling area took place in the middle Early Proterozoic(Ca.2250 Ma ago), resulting in the formation of the North China and Yangtze platforms and the paleo-Qinling oceanic taphrogenic trough between them. Based on their basement depths and cutting depths of their boundary, the plates within China can be divided into three categories,i, e. the lithospheric plate (with a cutting depth of over 100 km), the Mohorovicic plate (with a cutting depth of 45-100kg) and the crustal plate (with a cutting depth less than 45km). In most stages of the evolution of the North China platform and the Yangtze plaform, they were of mohorovicic type, and in few stages, they were of crustal one. This paper expounds that the Qinling tectonic belt is a complex polyeyclic and stacked breakup-collision orogenic belt. Its evolution underwent 5 stages and 9 phases, including 6 breakups and 6 collisions or compressions. There is enough evidence to show that the breakup depths of the Qinling belt are small,generally a few tens of kilometers, although it had been separated several times. The magmatic activity seems to be frepuent, but ultramafic rocks, especially dunities, rarely occur, and sheeted dykes are not developed either; therefore the ophiolite suite in this area is not typical. The authors call it the ophiolite-like suite, representing a transitional oceanic crust. In other words, the whole Qinling orogenic belt evolved in one single lithospheric plate, no true oceanic crust being involved. This view can be borne out from the history of mineralization in the area, which is integrated with the history of the geological evolution. The mineralizations are mainly of silver, gold and nonferrous metal(Pb, Zn, Mo, Cu)types, indicating the tranisitional feature of the crust. The gold mineralization is considered to have continued through the whole geological history, and gold deposits of major economic importance were formed in various geological stages under favourable geological and physiochemical conditions. The authors hold that the Shanyang-Lixian fault is not only a syngenetic fault belt that controls the Qinling Pb,Zn,Hg and Sb metallogenic belt but also an amalgamation belt between the Yangtze and North China platforms which collided in the Early-Middle TriassiC. Both the platforms as a whole began to drift northward from near the equator in the late Paleozoic. In the course of drifting, the Yangtz platform was nearing and collided with the North China platform and eventually they were fully amalgamated in the Middle Triassic. Because the platforms have belonged to the same lithospheric plate all the time, they did not belong to Gondwana in the south, nor belong to Laurasia in the north, but belonged to a third paleocontinent between the two supercontinents (e.g. possibly"paleo-Pacific Land").

       

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