Petrogenesis And Tectonic Setting Of Volcanic Rocks In Manketouebo Formation From Ta’erqi Area, Central Great Xing’an Range

This thesis studies the well-developed Manketouebo Formation volcanic rocks ofTa’erqi area in the major ridge of the middle section of Great Xing’an Range. Based onprevious studies, the rock assemblage, distribution rules and geochemistry ofManketouebo Formation volcanic rocks will be described, and the nature of magma source,petrogenesis and tectonic settings will be discussed in this paper, which will provide newinformation to researches of the volcanism and tectonic evolution history in Late Jurassicto Early Cretaceous in mid-southern Great Xing’an Range area.1.Geological characteristics of Manketouebo Formation volcanic rocksManketouebo Formation volcanic rocks, which are composed of rhyolite, rhyolitelava, tuff lava, ignimbrite and tuff with little intermediate-acidic volcanic rocks andsporadic sedimentary tuff and sedimentary rocks, are found at the study area, regionallydistributed along approximate NE-direction fault basin with a outcrop area of200km2,covering82%area of Mesozoic volcanic rocks in the study zone. And rhyolite is the majorrock type.2.Chronology characteristic of Manketouebo Formation volcanic rocksThe results of LA-ICP-MS ziron U-Pb dating are143.0±1.0Ma,141.0±1.0Ma and147±1.8Ma, revealing that Manketouebo Formation volcanic rocks are mainly formed inLate Jurassic-Early Cretaceous.3.Geochemistry characteristic of Manketouebo Formation volcanic rocksThe SiO2content is between69.22%and80.60%, and Na2O+K2O is between4.99%and10.67%. The ratio of Na2O/K2O is0.14-2.16, indicating the difference of K and Na issmall and relatively rich in K. Rittmann index are less than3. In SiO2-K2O diagram, most of the samples are belong to calc-alkaline-high-K calc-alkaline series. The value ofA/CNK is1.00-1.23and A/NK is1.03-1.29, the samples are belong to peraluminousseries. The volcanic rocks are also characterized by high silicon, full of alkali, short ofcalcium and magnesium and belong to high-K calc-alkaline peraluminous rock. Thesamples are all fall on the rhyolite in the TAS diagram.The chondrite-normalized REE paterns indicate that the volcanic rocks are enrichedin light rare earth elements and all strong negative Eu anomaly. ΣREE=30.87-307.60μg/g,LREE=46.30-289.68μg/g,HREE=2.17-37.92μg/g,LREE/HREE=3.70-16.17,δEu=0.14-0.80,and the value of (La/Yb)Nranges from2.40to22.18. The primitive mantle-normalizedtrace element spider diagrams shows right-dipping feature, which are similar with theupper crust mostly, revealing that the volcanic rocks enrich LILE (e.g., Rb, Pb, U, Th,)and deplete HFSE (e.g., Zr, Nb, Ta).These characteristics are almost in accord with thelow Ba-Sr rhyolite in Greater Xing’an Range.4.Characteristics and evolution of magma source of Manketouebo Formationvolcanic rocksThe characteristics of enrichment in SiO2, K2O and LILE and depletion in CaO, MgOand HFSE are similar to crustal magma. The strongly negative anomaly of Eu and theslight recess curves of HREE distribution patterns indicate that there are residual remainor crystal fractionation of plagioclases and amphiboles in the source area during formingof volcanic rocks. The positive values of εHf(t) and TMD2(501-905Ma) suggest theprimary magma were derived from depleted mantle or basic lower-crustal re-meltedmaterials, implying the source region may be the young crust accreted from depletedmantle in Early Paleozoic to Neoproterozoic.5.The tectonic setting of Manketouebo Formation volcanic rocksThe volcanics belongs to high-K calc-alkaline series with a few shoshonite andcalc-alkaline rocks, which is close to the characteristics of major element of post-orogenicgranite. In tectonic discrimination diagrams of trace element, all the samples fall intolate-orogenic and post-orogenic area. In R1-R2diagram, the samples distributed on COLGand POG area. And in Sr-Yb diagram of granite, they fall into low-Sr and High-Yb area (area Ⅴ). Combining the achievement of previous studies, it is concluded thatManketouebo Formation volcanic rocks were formed by the effect of the extensionalenvironment caused by the closure of Mongolia-Okhotsk.