The Genesis Of Volcanic Rocks Of Ying Cheng Group In Southern Songliao Basin

In recent years, with the discovering and exploiting of the natural gas in the volcanic rocks from Songliao Basin, the volcanic rocks in Songliao Basin have arose large attention of the geologist for the extensive prospect of oil gas exploration although they are in deep horizon and not too many driven well. The predecessors' research in volcanic rocks in southern Songliao Basin concentrate on lithologic characters,structure,volcano organization,the volcanic face division,the controlling action of volcanic rock to oil gas,being good reservoir and cap rock,but study degree is quite low at the aspects of chronology and geochemistry and there is difference regarding at basin's origin question.The thesis,through the studies of zircon U-Pb chronology,geochemistry and Hf isotope,disclosures the timing,the nature of source,the genesis and the dynamics background of volcanic rocks of Yingcheng group in southern Songliao Basin,and provides certain restriction on the formation of the Songliao Basin.1,The timing of volcanic rocks of Yingcheng group in southern Songliao Basinzircon U-Pb isotopic chronology is tested for the two rhyolite samples(ds7-1 and n47-1) from Yingcheng group in southern Songliao Basin using the method of LA-ICPMS. The core sample ds7-1 is picked from Deshen7 well at the depth of 3501 meters,the weighted average age of 206Pb/238U is 118±2.5Ma,MSDW=2.3.The core sample n47-1 is got from Nong47 well at the depth of 2211 meters, the weighted average age of 206Pb/238U is 113±2Ma,MSDW=0.90.So the timing of volcanic rocks of Yingcheng group in southern Songliao Basin is 113-118 Ma(late of the Lower Cretaceous)2,Hf isotope of the volcanic rocks of Yingcheng group in southern Songliao BasinThe zircon 176Hf/177Hf ratios of the two samples of Yingcheng group in southern Songliao Basin are comparatively high, the weighted average are 0.282909±0.000016(2σ,n=20)and 0.282970±0.000025(2σ,n=20.The weighted average ofεH(ft)are 7.32±0.55 and 9.31±0.87. The ranges of Hf two stage model ages (TDM2) are 556-876Ma and 374-869Ma.The zircons of the volcanic rocks of Yingcheng group in southern Songliao Basin have plusεHf(t)results ,the two stage model age are from Neoproteozoic to Phanerozoic.This indicates that the source of the rocks is the new crust which growth from the depleted mantle in Neoproteozoic and Phanerozoic ,they melted in Mesozoic and formed the volcanic rocks of Yingcheng group.3,Petrology,geochemistry and genesis of the volcanic rocks of Yingcheng group in southern Songliao BasinThe volcanic rocks of Yingcheng group in southern Songliao Basin are divided into three types because of the characters of geochemistry,1-type rhyolite,2-type rhyolite and dacite. 1-type rhyolite: SiO2 contents range from 71.62% to 75.76%, Al2O3 contents range from 10.88% to 12.92%, entire iron contents range from 2.24% to 4.44%, MgO contents range from 0.03% to 0.29%, CaO contents has a large change scope range from 0.08% to 0.92%, Na2O,K2O content respectively 0.54%-4.63%,4.3%-9.34%, TiO2,MnO,P2O5 have low content 0.22%-0.32%,0.06%-0.18%,0.01%-0.04%. 1-type rhyolite belongs to sub-alkaline on the TAS diagram and high K calkaline on the SiO2- K2O diagram.The variable range of NK/A and Na2O+K2O are 0.85-1.08 and 7.8%-9.8% ,so 1-type rhyolite is similar with A-type granite.The rare earth elements are higher than ordinary granite, has obviously negative Eu anomaly. The chondrite-normalized REE distribution pattern is slightly right V-type incline. HFSEs for example Zr,Hf,Nb content is generally high, Sr,Ba content is low.1-type rhyolite has obviously negative anomaly of Ba,Sr,P,Ti on the primitive mantle standard spider diagram.The 10000Ga/Al ratios are 3.46-5.59 (bigger than 2.6), Zr+Nb+Ce+Y average content is 1319.456 ppm (bigger than 350 ppm ),all these characters demonstrate 1-type rhyolite is A-type granite. SiO2 content is 71.62-75.76%,Al2O3 content is 10.88-12.92%,low Nb/Ta ratios(average 13.78) and slightly negetive Nb and Ta anomaly, light– heavy rare earth fractionation,not obvious LaN/YbN (mean value =9.78) <10,extreme obviously negetive Eu anomaly(Eu/Eu*=0.27),all these characters indicate 1-type rhyolite is A2-type granite.2-type rhyolite: SiO2 contents range from 72.9% to 73.6%, Al2O3 contents range from 13.96% to 15.63%, entire iron contents range from 0.6% to 1.32%, MgO contents range from 0.06% to 0.31%, CaO contents has a large change scope range from 0.22% to 1.09%, Na2O,K2O content respectively 3.14%-4.27%,4.2%-5.05%, TiO2,MnO,P2O5 have low content 0.18%-0.3%,0.04%-0.07%,0.03%-0.06%. 2-type rhyolite belongs to sub-alkaline on the TAS diagram and high K calkaline on the SiO2- K2O diagram.The variable range of NK/A and ACNK are 0.72-0.83 and 0.98-1.28,the rare earth elements concent 151.065-236.982ppm,LREE is higher than HREE and the average LREE/HREE is 10.95. (La/Yb)N range from 10.76 to 14.29(average 12.01), has obviously negative Eu anomaly(δEuN= 0.41-0.47) . The chondrite-normalized REE distribution pattern is slightly right V-type incline. LILEs, Rb,Th,U for example,are rich; K,Zr and Hf are lightly rich; Ba,Nb,Ta,Sr,P and Ti are comparatively low. All these characters indicate 2-type rhyolite is I-type granite.Dacite: SiO2 contents range from 61.99% to 65.08%, Al2O3 contents range from 15.68% to 16.45%, entire iron contents range from 3.23% to 4.53%, MgO contents range from 0.82% to 2.36%, CaO contents range from 2.83% to 3.94%, Na2O,K2O content respectively 3.55%-4.38%,1.34%-3.34%, TiO2,MnO,P2O5 have low content 0.56%-0.61%,0.05%-0.15%,0.18%-0.20%. Dacite belongs to sub-alkaline on the TAS diagram and high-middle K calkaline on the SiO2- K2O diagram.The variable range of NK/A and ACNK are 0.52-0.69 and 0.80-0.86.The total quantity of rare earth elements is 98.669-122.082ppm, the light rare earth content is higher than the heavy rare earth content, the LREE/HREE mean value is 12.57, (La/Yb)N is 15.15-18.78.Dacite has these characters that slightly negative Eu anomaly(δEuN=0.71-0.81),high Ba,Sr content, low Y,Yb content,high Sr/Y,Sr/Yb ratios.Dacite has positive Ba,Sr anomaly,slightly negative P,Ti and HFSEs(Nb,Ta for example) on the primitive mantle standard spider diagram. All these characters indicate dacite is I-type granite.4,Genesis mechanism of the volcanic rocks of Yingcheng group in southern Songliao BasinDacite is like adakite because of these characters SiO2≥56%,Al2O3≥15%,low MgO content(<3%), slightly negative Eu anomaly, low Y(≤18μg/g),Yb(≤1.9μg/g)content,high Sr content(>400μg/g)and Sr/Y ratio(>40),low HFSEs(Nb,Ta for example), high(La/Yb)N ,light and heavy rare earth element strong differentiation,obvious positive Sr anomaly.The low Mg# (14.77-36.46) indicate local adakite is formed by the partial melting of the underplating lower crust.According to the contents of Sr,Y,dacite belongs to high-Sr and low-Yb type and its melting residual source is dominant by amphibole + garnet + pyroxene and maybe a small amount of plagioclase,it is formed in the high pressure environment at least at the depth of 40km.2-type rhyolite belongs to low-Sr and high-Yb type and it is formed in a lower pressure environment at the depth of normal crus(tabout 30Km).1-type rhyolite belongs to very low-Sr and high-Yb type is A2-type granite and plagioclase is the residual face in the source which indicate the depth of the source is shallower(<30Km)The major element of these three kinds of volcanic does not change correspondingly as the SiO2 changing,trace element ratios are inconsistent as well as these volcanic rocks output in three different places which indicate three kinds of volcanic rocks of Yingcheng group are not the result from the same origin magmatic differentiation,but the different depthes of the source of volcanic rocks. So the genesis mechanism of the volcanic rocks of Yingcheng group is rocks in different depths of the crust occured partial melting in the same time and formed different kinds of magma in the Lower Cretaceous.High-Sr and low-Yb type dacite is formed in the high pressure lower crust at least at the depth of 40Km. Low-Sr and high-Yb type 1-type rhyolite and very low-Sr and high-Yb type 2-type rhyolite are formed in the low pressure middle-upper crust, the former's depth is 30Km,the latter's is smaller than 30Km. These different depthes' magma erupt from different cauldron in different places and form dacite and two kinds of rhyolite which have different geochemistry characters.5,The tectonic setting of the volcanic rocks of Yingcheng group and the confinement to Songliao Basin.The characters of the Rittman index,Gedi Nepal index and the rare earth elements indicate local volcanic rocks output in oroganic zone and islang arc.Local volcanic rocks have the characters of enriching light rare earth element and LILEs,dificiting HFSEs and Nb,Ti,P which like the post-collision granites.The large occurrence of post-collision middle-acid igneous rocks indicates the turning from convergent to extension.The occurrence of bimodal volcanic rocks,A-type granite,metamorphic core complex and A2-type granite in this study indicate it is intensive extension setting in northeastern China in the Low Cretaceous.Three kinds of volcanic in this paper are the results of this extension setting which indicate the Songliao Basin is in the post-collision extension setting of organic zone,and the tectonic system turns from convergent to extension. Under the tectonic setting of extension,the rocks in different depths occur partial melting by the magma underplating and form three different kinds of volcanic rocks of Yingcheng group.