西南三江造山带火山岩-构造组合及其意义

Volcanic Petrotectonic Assemblages in Sanjiang Orogenic Belt, SW China and Implication for Tectonics

岩石构造组合是指表示板块边界或特定的板块内部环境特征的岩石组合。中国西南“三江”造山带的火山岩可划分为五种火山岩 -构造组合 :洋脊型 /准洋脊型组合 ,岛弧及陆缘弧组合 ,碰撞型组合 ,碰撞后组合及陆内拉张型组合。阐述了各种火山岩 -构造组合的特点及构造含义。对在造山带火山岩岩石 -构造组合分析中经常遇到的一些问题 ,如“构造岩片”研究方法、地球化学判别图解的使用条件、准洋脊型火山岩组合的构造含义、蛇绿岩带 -火山弧的成对性、岩浆作用的同步性和滞后性、以及火山岩的深部“探针”作用等问题进行了讨论。

Sanjiang Orogenic Belt is located geographically in the area of Jinshajiang, Lancangjiang and Nujiang (abbreviated from the “three rivers area”), and tectonically at the junction between the Himalaya Tethyan tectonic domain and the Pacific tectonic domain. It is one of the key areas to understand the Tethyan evolution, Indian Eurasia collision and the uplift of Tibet Plateau and its eastern extension. Various volcanic rocks of Proterozoic to Cenozoic age occur in Sanjiang Orogenic Belt. The majority of volcanic rocks, however, formed during the Tethyan and post Tethyan stages, i.e., from early Carboniferous to the Cenozoic. Volcanic petrotectonic assemblages as geological records and a lithoprobe play an important role in understanding tectonic evolution and corresponding deep processes of the Sanjiang area. Five types of volcanic petrotectonic assemblages in Sanjiang Orogenic Belt have been recognized as follows: Oceanic assemblages including MORB/Para MORB(or MORB LIKE) assemblage and OIB assemblage, island arc and continental marginal arc assemblage, collision related assemblage, post collisional assemblage and intracontinental assemblage. Fig 1 shows a frame of their spatial and temporal distribution. Sanjiang MORB and para MORB assemblages occur in four major suture zones, being closely associated with peridotite (or serpentinite), gabbro, diabase, radiolarian chert or deep water limestone, and sometimes accumulates in ophiolites. Geochemically, Sanjiang MORB are mainly E type MORB, although they have the lowest K 2O content and (La/Yb) N ratio among all types of volcanic rocks in Sanjiang Orogenic Belt. Para MORB is mainly tholeiites and geochemically more enriched in K 2O, LREE and incompatible elements than E MORB but less enriched than OIB. Sanjiang OIB occurs so far only in Changling Menglian belt and is associated with overlying limestone and sometimes underlying MORB/para MORB. They are mainly alkaline olivine basalts and the most enriched type in Na 2O+K 2O, LREE and incompatible elements among all types of oceanic volcanic rocks in Sanjiang Orogenic Belt. Island arc and continental marginal arc volcanic petrotectonic assemblage in Sanjiang orogenic belt is of high variety, although they are mainly composed of basalts, basaltic andesites, andesites, dacites and rhyolites, belonging to calc alkaline or island arc tholeiitic series. Arc volcanic rocks in Sanjiang Orogenic Belt formed in various environments, such as oceanic island arc, continental island arc, continental marginal arc, fore arc, main arc, back arc and so on. Among them, it was particularly important to recognize tensional arcs in Sanjiang Orogenic Belt. A typical example of tensional arcs is Yidun arc formed in Late Triassic, which is characterized by an intra arc rifting with a bimodal volcanic suit. A large Kuroko type Pb Zn Ag ore field was developed within the intra arc rifting basin (see Fig.7 in text). The southern end of Yidun Arc, however, became a compressive one associated with many porphyry type copper deposits. It is also amazing to see a special type of volcanic rocks, which contain strong subduction zone components (SZC) but formed in post collisional intra continental environments. We therefore called it 'post collisional arc volcanic rocks' or 'delayed arc volcanic rocks'. Condie et al(1989), Aldrich et al(1986) reported similar cases in Kaapvaal of South Africa and in Rio Grande rift, respectively. A possible explanation for the delay of arc type volcanism and for the formation of post collisional arc volcanic rocks was presented ( see Fig.8 in text). It is apparently that geotherm in the source region during the subduction was too low to reach the solidus of source rock. The subducted or collapsed slabs, however, might react with surrounding mantle and become a new source rock for magma generation afterwards. In post collisional stage some kind of geological events such as upwelling, convection, intra continental convergence/subduction might cause generation of magmas, whi