Determination Of Chencai Subduction Accretion Complex

A suite of metamorphic rock group is "skylight" type in Southeast Zhejiang province, respectively is comprised by Badu Group, Chencai Group, Longquan Group and Longyou Group.The metamorphic rocks in the area are mainly distributed in the South of Zhejiang, northern Fujian, south of Jiangxi and Yunkai mountain in the Southeastern of China. Geological circles generally think itof a part of the Cathaysia Block, and will be divided into two tectonic layers. In Zhejiang Province, BaduGroup is the lower tectonic layers, and Chencai Group, Longyou Group and Longquan Group are the upper. The mainstream view is that the Cathaysia Block consists of this metamorphic rocks had collided with the Yangtze block as early as the mid Neoproterozoic. Soon after, with the break-up of Rodinia supercontinent, South China plate turned into the stage of the rift.The intracontinental orogenic movement in Early Palaeozoic made this two Block to the uniform South China plate. But some other scholars think that Chencai Group of Zhejiang province in general is a suit of disordered formation and is actually subduction accretionary complex composed of rock blocks and matrixes. This opened the prelude of a new round debate about the tectonic study of South China in pre Devonian.In order to solve the arguments, this paper has carried on the research to Chencai Group and the corresponding "Strata". In this paper, the metamorphic rocks of Zhuji, Longyou and Longquan area of the so-called "upper tectonic layer" are selected as the research object, especially to Zhuji area. Through detailed field investigation, key coding method, and in the dew of large quantities of base and magnesian metamorphic rock block as the focus, this paper carrys out detailed geology, petrology, geochemistry and zircon geochronological study.Research shows that the subduction accretionary complex outcropped in Zhuji area consists of five rock blocks as oceanic island or seamounts, oceanic ridge, intra oceanic arc, accretionary arc,and palaeo continental crust and two matrixes as volcano flysch and sedimentary flysch. The contact is construction between the rock block and the matrix. The rock assemblage of the oceanic island or seamount rock block is amphibolite and marble with the protoliths equivalent to basalt and carbonate. The crystallization and metamorphic age of the amphibolite ae 857Ma and 421 Ma and the metamorphic age of the marble is 425-440Ma. The protolith of the amphibolite is alkaline basalt primarily. The amphibolite is characterized by high Ti, alkali, E REE, {La/Yb) N and without the depletion of Nb, Zr, Hf and Ti. The geochemical characteristics is similar to the modern oceanic island alkaline basalt. The rock assemblage of the mid oceanic ridge rock block consists of hornblendite, amphibolite(or amphibolite gneiss) and plagioclasite with palimpsest crystal texture. The rocks are characterized by low K, E REE, high Ti and the depletion of LREE. The ratio of the Nb/La, Hf/Ta, Hf/Th are similar to N-MORB. The metamorphic age of one of the rocks is 432Ma. The rock assemblage of the intra oceanic arc rock block consists of (pyroxene) hornblendite and amphibolite (or amphibolite gneiss). The main rock geochemical parameters are between VAB and N-MORB. The distribution model of the rare earth elements of the rocks are similar to N-MORB and T-MORB. But the trace elements of Nb, Ta are negative anomaly. The metamorphic age of the rock is 441 Ma. The accretionary arc rock blocks include olivine pyroxenite rocks and acid-acid intrusive rocks. The former is poor in Si, K, Na, and rich in Fe,Mg. The rock may be formed by the separation and crystallization of basaltic magma.The geochemical characteristics are similar to the island arc tholeiitic rocks. The rock assemblage of the latter consists of granodiorite, tonalite, monzonitic granite and trondhjemite, belonging to the TTG+GQM assemblage. The geochemical characteristics of this rock is similar to the volcanic arc granite. The crystallization age of this rocks are 850-780Ma, and the metamorphic age is 435Ma. The palaeo continental crust debris is mainly composed of granulite, leucoleptite, quartzite, gneiss rock. The youngest detrital zircon age is generally greater than 18Ga,and the age of metamorphic zircon rims is 441 Ma. The rock assemblage of the volcano-flysch matrix with strong deformation consists of the amphibolite, leucoleptite and quartzite. The protolith of the amphibolite is calc alkaline tholeiitic basalt and basaltic andesite. The petrological and geochemical characteristics are similar to the continental margin arc basalts. This indicates that the rocks formed in the near arc basin. The rock assemblage of the sedimentary flysch matrix consists of gneiss, granulite,leucoleptite and schist. The rocks experienced strong ductile deformation with various kinds of deformation structures. The geochemical study shows that the protolith debris of the rocks are mainly from continental margin or oceanic arc. The youngest detrital zircon age of the rocks is 601 Ma, and the age of the metamorphic accretion edge of the zircon is 420-440Ma.Research shows that the subduction accretionary complex outcropped in Longyou area is mainly composed of various types of ultrabasic rocks, basic-mafic metamorphic rocks, HP-UHP metamorphic rocks and flysch matrix. The ultrabasic rocks includes serpentinized peridotite, pyroxene peridotite and hornblende peridotite. The genesis of this rocks is not the same. Some of them belong to mantle peridotite with tectonic intrusion, some formed by the crystallization of mantle ultrabasic magma, and some formed by the fractional crystallization of the basalt magma. But their geochemical characteristics is a close relative of the island arc. The metamorphic age of individual peridotite is about 415Ma. The basic-mafic metamorphic rocks are the main rocks of Longyou accretionary complex. Their lithology are mainly in different size plagioclase amphibolite with the protolith of basic and ultrabasic igneous rocks. Geochemical characteristics difference of different rock block is bigger. Based on the REE distribution model, the rocks are divided into "N-MORB", "T-MORB" and "E-MORB" three types. The protoliths of the "N-MORB" is mainly born in the intra-oceanic arc, the "T-MORB" is mainly born in the transition of mid-oceanic ridge and the volcanic arc, and the" E-MORB " type rock is born in the enrichment of mid-oceanic ridge, epicontinental arc and intraplate environment. Individual rock metamorphic age is 424Ma. The lithology of the HP-UHP metamorphic rocks is garnet hornblendite. The typical degeneration structure developed in the rock. The temperature and pressure conditions respectively from 668 to 821 C and 2.6 to 3.3Gpa are obtained from the residual mineral temperature-pressure estimation. This shows that the rocks have experienced UHP metamorphic stage. The forming age of the protolith is 829Ma, and the metamorphic ages is 451 to 454Ma. The lithology of the matrix of Longyou accretionary complex are mainly for all types of gneiss, granulite and schist rock containing in carbon and rich in Al. The rocks experienced strong ductile deformation. Geochemical studies reveal that these matrix are mainly formed in the active continental margin. The youngest detrital zircon age of the rocks is 661 Ma, and the age of the metamorphic edge is 442Ma.Research shows that the subduction accretionary complex outcropped in Longquan area consists of ultrabasic rocks, metabasic rocks, quartzite, marble and matrix. The lithology of the ultrabasic rocks are peridotite and augite peridotite. The ratio of m/f of this rocks ranged from 5.16 to 7.16, and Mg# (83.93-87.86) is relatively low. The magma may originated in the mantle wedge by fluid metasomatism. The rocks is enrichment of LREE, and depletion of high field strength elements Nb, Ta, P, Ti. The petrology and geochemistry characteristics are similar to "SSZ" type ophiolite. The lithology of the metabasic rocks are mainly amphibolite. The protolith may be equivalent to the alkali basalt and tholeiite basalt. The E REE of partial rocks is low, depletion of LREE and the geochemical characteristics is similar to N-MORB. The ?REE of the other part of the rocks is high, enrichment of LREE and HFSE Nb, Ta, and the geochemical characteristics is similar to that of OIB. Through regional comparison, the protolith of the metabasic rocks can be formed mainly in Neoproterozoic with age from 756 to 853Ma,and metamorphed in the early late Paleozoic with age from 340 to 380Ma. The chemical composition of the quartzite are silicon and iron. The E REE and ratio of Ti/Nb of the rocks is similar to chemical deposition of siliceous rocks. The rock sedimentary diagenetic age is 799Ma. The protolith of the marble is limestone, argillaceous limestone and dolomitic limestone. The carbon oxygen isotope compositions of the marble indicates that it belongs to the marine sedimentary environment. The matrix of Longquan accretionary complexes consists of various granulite and schist. The local profile showes grain sequence changes of the matrix. This reflects the protolith is a flysch formation. The matrix rocks are strong deformation. The youngest detrital zircon age is 631 Ma, and the metamorphic age is as early as possible in 410 to 420Ma.The results reveal that the metamorphic rock series above three regions is really disordered rock flakes in outcrop, no obvious "marker" with regional extension, and is the distribution of "rock block and matrix "withductile fault for the sector. The geochemicalstudy about the basic and magnesian metamorphic rocks also shows that they are complex and varied geochemical characteristics," and most tectonic environment of them are the oceanic crust or subduction zone. The research of the rock chronology shows that the rock blocksmainlyformed in the early and mid Neoproterozoic, the oldest rock block formed in the Paleoproterozoic, and the matrix formed in the late Neoproterozoic to early Paleozoic. Either the matrix or block, they were metamorphosed in the early late Paleozoic. The rock blocks with different age isno characteristics with banding property in space, but disordered mixed. This is significantly different with Badu Groupof the "lower tectonic layer"supposed to be good stability characteristics. The studies show that this suit of metamorphic rocks as the "upper tectonic layer" of the Cathaysia Block in Zhuji, Longyou, Longquan area is subduction accretion complex, named "Chencai subduction accretion complex".Through regional comparison, we find that there are a lot of evidence of subduction accretionary complex in Wuyishan area,the south of Jiangxi Province and Yunkai-dashan area in the margin and hinterland of the Cathaysia orogenic belt in addition to the southeast of Zhejiang province. They constitute the giant subduction accretionary complex belt in Cathaysia.The redefinition of Chencai subduction accretionary complex has important implications for the pre Devonian geological tectonic pattern and evolution in South China. The research results indicate that there is no unified continental craton in South China in the late Proterozoic to early Paleozoic. The Southern China plate is not composed of collision with the Cathaysia Block and the Yangtze block in the early and mid Neoproterozoic. It is composed of a plurality of micro continental blocks, island arc, accretionary complex through the method of accretionin early late Paleozoic.So the South China Caledonian orogenic belt is essentially accretionary orogenic belt, not intracontinental orogenic belt. In addition, the determining of Chencai subduction accretionary complex has certain instructive significance in Zhejiang regional prospecting and deep prospecting.