腾冲地洼区—一个复合型地洼区

腾冲地区是由元古代的地槽褶皱带,经古生代—三叠纪的地台体制,至侏罗纪开始构造-岩浆活化作用形成的地洼区。该区地洼阶段的演化及成因机制分为二个阶段。早期(侏罗纪-早第三纪)以花岗岩多次侵入为特征。这些花岗岩的主元素、微量元素及同位素特征均可与中国东部地洼花岗岩相对比。此阶段的地台活化作用源自该区的深部活动过程。晚第三世纪开始,该区的大地构造演化机制发生重大转折。巽他陆块与印度陆地的碰撞控制着该区的活化作用,形成广泛的中—基性火山岩喷发,这些火山岩的地球化学研究证明其构造背最及成因均与火山弧岩石相似。因此,从成因机制角度,腾冲地区属一个复合型地洼区。

THE TENGCHONG DIWA REGION——A COMPOUND TYPE OF DIWA REGION

The Tengchong Diwa region is located at the southwestern edge ot Yunnan Province, China. The geosynctine folded basement in this area is constituted by the Gaoligong mountain metamorphic belt which consists of gneisses, migmatic gneisses, migmatites, migmatic granites, schists and plates. Late Paleozoic and Triassic dolomites, limestones, bioclastic limestones, sandy conglomerates, sandstones and shales constitute the platform sedimentary cover, Since the early Jurassic, the Tengchong region has been uplifted and is lacking in sediments except local Tertiary and Quaternary nonmarine sediments.In the early Jurassic, the platform was activated. During teh early Diwa stage (from Jurassic to early Tertiary), granites intruded in early-and late-Yan-shanian and early-Himalayan movements(～140—120Ma,～70Ma and ～50-60 Ma re-spectively). The granites are characterized by high K2O+Na2O,SiO2,DI and(~(87)Sr/~(86)Sr)i, low CaO and Mg/Fe ratio. Chondrite-normalized REE patterns show that (La/Yb)_o and Eu/Eu# deease from early to late-Yanshanian granites. These geochemical characterictics reveal that the granite in the Tengchong region are similar to Diwa graniets in eastern China, They are generated under a tensional setting, Since the late Tertiary, magmatism in the Tengchong region has changed to eruption of basic and intermediate volcanic rocks. The volcanic rocks include basalts, basaltic andesites, andesites and dacites which are of high patassium series.Thei(?) Chondrite-normalized REE patterns show LREE-enriched and small or nonegative Eu and Ce anomalies.There are negative Nb,P and Ti anoma lies in the incompatible element abundances normalized to primordlal mantle,It is evident that the genesis and tectonio setting of the volcanic rocks in the Tengchong region are generally consistent with that of island arcs and volcanic arcs.The geotectonic evolution constrains the geochemicalcharacteristics, genesis and tectonic settings of the magmatic rocks in the Teng thong region. The geotectonic regime has resemblig features in Tengchong and eastern China from Jurassic to early Tertiary, The incompatible clement enrichmet (mantle meta-somatism) results in the increase of the heat flux in the upper mantle, the crust's tension and block-faulting. Under this tectonic setting, the melting of the upper crust yields granitic magmas. The geotectonic regime in the Tengchong region takes a turning point from late Tertiary beginning. The collision between Indian continent and northern Sundland began about 20 Ma ago. The involvement of subducted oceanic crust and recycled continental material into the mantle wedge triggers volcanic eruptions in the Tengchong region.The geotectonic regime evolution of the Tengchong Diwa region has undergone two stages. In the early Diwa stage (from Jurassic to early Tertiary), the platform activation is constrained by the internal process (mantle metasomat-ism). The platform activation is called "active activation". However, the plat-form activation in the late Diwa stage (since late Tertiary) is controlled by the external process (collision between Indian continent and Sundland). The plat-form activation in this stage is assigned to "passive activation". Therefore, the formation and evolution of the Tengchong Diwa region cannot be explained by a single process. The Tengchong Diwa region is best interpreted as a new type of Diwa region—a compound type of Diwa region