Geochemistric Characteristic Of Rhyolite And Trachyte From The Early Cretaceous In Southern Jiangxi

The late Mesozoic volcano rocks are widely developed in the southern area of Jiangxi Province, which is just the field of my research. The rocks formed in three different periods which are respectively the late, the middle and the early periods, and they formed different rock assemblages of volcanic series. The volcanic series distributed in Banshi basin, Cai Fang basin and Sanbaishan basin, With “K calc-alkaline rhyolite- a combination of alkaline trachyte characterized. According to the characteristics of lithology and lithofacies, the volcanic series belongs to Wuyi group and can be divided into three stratigraphic units: the lower part is purplish red conglomerate, sandstone, siltstone folder acidic volcano rock and can be called “Shaungfengling Formation”; the middle is rhyolitic tuff, rhyolite ignimbrite dominated, called “Ehuling Formation”; the upper is mottled sandstone, shale, conglomerate acidic volcano rock, called “Shixi Formation”.Wuyi group of rhyolite, in TAS figure falling into a series of the alkaline rhyolite class scope, on the diagram of K2O- Na2 O fall into scope of high potassium to pledge, in K2O/Na2O- Si O2 diagram into calcium alkali series, is high in potassium calc-alkaline series. The main elements of Rhyolite showed high silica rhyolite, high Potassium, high alkali, low aluminum, magnesium, calcium, Poor sodium. The content of Al2O3 is 12.9%~14.3%(average 13.6%), ACNK 0.93~1.21(average 1.01), in the A/NKC-A/NK diagram, most of it falls into the scope of a metaluminous, some falls into the scope of strongly peraluminous, which belongs to metaluminous to peraluminous rock series. The total rare earth of Rhyolite is high, 212 ~ 253 by 10-6(average 262 x10-6), light rare earth concentration, obviously LREE/HREE is 8.60 ~ 13.4(average 10.4). Significantly, light and heavy rare earth fractionation(La/Yb) N is 8.68 ~ 15.7(average 11.5), rare earth distribution curve exists obvious Eu negative anomaly. Enrichment of large ion lithophile elements(LILE, such as Rb, Th, U, and K) and LREE, loss of Ba, Sr, Ti, P, Ta, Nb, etc., are similar to the typical continental crustal remelting type granite. The High contents of Rb, Th are displayed with peraluminous rocks characteristics, Rb is 173×10-6~274×10-6( average 207 ×10-6), Th is 20.3 ×10-6~ 24.9 ×10-6( average 21.8 ×10-6), the Si O2-P2O5, Th-Rb, Y-Rb relationship diagram showing evolutionary trends in S-type granites. In Isr-εNd illustration, rhyolite crustal evolution along the distribution line, and causes the upper crust of flour rhyolite stone basin, southwest Guangxi rhyolite, consistent with mountain rock Zoujiashan ore deposit investment point range, significantly different from the Yixian Formation in western Liaoning lower crust causes Daxingzhuang rhyolite, Ju early Cretaceous rhyolite. In(206Pb/204Pb)i-(207Pb/204Pb) i and(206Pb/204Pb) i-(208Pb/204Pb) i diagram, it also falls into the upper crust causes range.Wuyi group trachyte on the TAS diagram all fall within the scope of alkaline trachyte with the characteristics of potassium-rich, alkaline-rich, high-alumina, sodium poor, calcium, magnesium. A / CNK, 0.91 to 1.06( average 0.97), in the A/NKC-A/NKdiagram, most of the samples in the sub-alkaline metaluminous projection area, individual investments in weakly peraluminous area, belongs to metaluminous to weakly peraluminous series. Trachyte total rare earth is slightly lower than the rhyolite, ΣREE = 200 × 10-6 ~ 227 ×10-6( average 214 ×10-6), light rare earth enriched significantly, LREE / HREE = 9.50 ~ 14.1( average 12.2). LREE fractionation is obviously,(La / Yb) N of 13.0 to 18.6(average 15.1). The REE graph, with obvious positive Eu anomalies, high Ba, Zr content, Sr, Nb anomalies Valley was not significant, suggests that magma experienced a lower level of plagioclase fractional crystallization or partial melting process plagioclase residual rate small. Enriched in LILE elements(LILE, such as Rb, Th, U, and K) and LREE, losses of P, Ti, Ta, Nb, etc., also shows the continental crust source characteristics. Trachyte rare earth distribution curve, the spider diagram types and fractional crystallization of basaltic magma formed Gannan yellow port syenite, syenite Gan Nantucket back significantly different, with lower thickened continental crust formed by partial melting of the lower crust Yixian Formation in western Liaoning Jianchang trachyte consistent, unlike the formation of the crust-mantle mixing Zhejiang Huangyan Wang Hai Gang syenite. Its distribution is also distributed along the line of crustal evolution in Isr-εNd illustration; in(206Pb/204Pb)i-(207Pb/204Pb)i and(206Pb/204Pb)i-(208Pb/204Pb)i diagram, showing on the causes of the crust.Rhyolite and trachyte in La / Sm-La map, La / Yb-Yb, ISr-Si O2 diagram, εNd(t)-Si O2 diagram in substantially parallel to the partial melting trend line, indicating that the formation mechanism of partial melting of its main. The K2O/Na2 O of Rhyolite and trachyte, Al2O3 / Ti O2, K2O/Ti O2, K2O/P2O5 and Ca O/Na2 O there exists significant difference ratio, in the main, huck diagram of trace element, except TFe O, Mn O, ALK, K2 O, Ba elements such as the evolution trend of certain continuity, the evolution trend of the other elements exist obvious difference. In the Si O2-A12O3 illustration, rhyolite samples are located in the metasediments, rough surface rock samples located near the water-poor basalt; In(La / Yb) N / δEu illustration, rhyolite falls into the crust area, part of the rough rock falls into the crust area, and some fall into the crust-mantle source region, indicating that the material may be added to the mantle; in Rb / Sr-Rb/Ba illustration, most fall into the sand rhyolite rocks area, the individual falls into mudstone area, all of the trachyte fall into the clastic zones; In ratio spider diagram of trace element, rhyolite and on the earth’s crust formation of white stone basin rhyolite, southwest rhyolite elements on similar ratio, trachyte and granite rocks from the dabie mountains, western liaoning the root cause of the lower crust in yixian rhyolite elements on similar ratio; and δ18O values rhyolites(9.83 ‰ ~ 10.87 ‰, average 10.28 ‰) higher than trachyte(6.28 ‰ ~ 7.91 ‰, average 7.23 ‰), the source explained trachyte deeper than rhyolite. However, rhyolite and trachyte have similar Sr-Nd-Pb isotopic composition, characteristics inconsistent with the former, which may be the result provenance component exchange. The TDM2 of Rhyolite is1542 ~ 1581Ma(average 1553Ma), the TDM2 of rough rock(1462 ~ 1476 Ma, average 1472Ma), less than China Block basement metamorphic age of 1800 ~ 2200 Ma, expressed that mantle materials may be added.Rhyolite and trachyte, in the identification chart of Al2O3-Si O2, TFe O /(TFe O + Mg O)-Si O2, TFe O-Mg O,(TFe O + Mg O)-Ca O, R1-R2, Rb-(Nb + Yb) and Nb-Y, etc., most fall into extensional tectonic setting, some fall into the compressional tectonic environment, combined with the occurrence of late Jurassic in southern squeeze a lot of geological facts inferred rhyolite, trachyte should belong to strong compression background extensional tectonic environment.To sum up, in southern jiangxi province, wuyi group under the background of rhyolite is the product of thickened crust ispartial melting, trachyte is the product of lower crust is partial melting.