Chronology And Geochemistry Of Mesozoic Volcanic Rocks In Southeastern 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. Although some scholars have investigated the Mesozoic volcanic rocks in Songliao Basin detailedly,the chronology and stratum contrast of the volcanic rocks still has diverges,the tectonic setting of Songliao Basin still has disputes.The thesis through the study of chronology,geochemistry and Hf isotope of zircon,in order to discuss the timing and crust source of the volcanic rocks in Songliao Basin.1 Chronology of the volcanic rocksZircon U-Pb isotopic chronology is tested for the dacite(GW04-555)and the rhyolite(GW04-5-2,GW04-5-5) from Yingcheng formation using the method of LA-ICPMS.The sample GW04-5-2 is collected in Jiutai ,²⁰⁶Pb/²³⁸U age is 117.6±0.5Ma,the sample GW04-5-5 is also collected in Jiutai,²⁰⁶Pb/²³⁸U age is 106.0±1.6Ma,the sample GW04-555 is collected in Jingyue,²⁰⁶Pb/²³⁸U age is 119.90±0.9Ma.2 Petrology and geochemistry of the volcanic rocksThe volcanic rocks in southeastern Songliao Basin belong to sub-alkaline series on the total alkali-silica diagram and show the evolution trend of calc-alkaline series.The SiO₂ contents of the rhyolite in Jiutai range from 73.99% to 81.29%,Al₂O₃ contents range from 9.32% to 13.75%,the contents of alkali,K (average 6.91%)and Na (average 2.17%)are high,K₂O/Na₂O average ratios 3.86%.The contents of MgO(0.01%⁰.16%),CaO(0.05%⁰.52%)and TFe2O3(0.04%⁰.26%) are low.NK/A is 0.85⁰.93,ACNK is 1.05¹.11.∑REE abundances range from 70.17μg/g to 110.66μg/g ,LREE/HREE average ratios 5.75,(La/Yb)N average ratios 5.25.δEu is 0.08⁰.10,it has obvious negative Eu anomaly.The chondrite-normalized REE distribution pattern is slightly right V-type incline,is charactered by low Ba,Sr and high Rb/Sr ratio,HFSE(e.g. Zr,Nb,P,Ti)contents are low.On the primitive mantle-normalized spider diagram,the obvious low elements are Ba,Sr,P,Ti,the elements of Rb,Th,U,K are comparatively rich.The SiO₂ contents of the dacite in Shibeiling range from 62.90% to 63.67%,Al₂O₃ contents range from 16.75% to 18.35%,the contents of MgO,CaO and TFe₂O₃ are 0.01%⁰.16%,0.05%⁰.52%,0.04%⁰.26%.The contents of K and Na are 4.21%⁴.70% and 2.40%².81%,Na₂O/K₂O average ratios 1.73.NK/A is 0.56⁰.61 , ACNK is 0.93¹.08.∑REE abundances range from 90.33μg/g to 99.71μg/g,LREE/HREE average ratios 10.38,(La/Yb)N average ratios 14.42,there is no negative Eu anomaly.The chondrite-normalized REE distribution pattern is LREE enrichment incline,the rocks abundant in Sr,the Y,Yb contents are low,Sr/Y ratios is 70.56.On the primitive mantle-normalized spider diagram,the HFSE(e.g. Ta,Nb,Yb,Ti)are comparatively low,the LILE(e.g. Ba,Sr,P,Ti)are rich. 3 Hf isotope of the volcanic rocksThe ¹⁷⁶Hf/¹⁷⁷Hf ratios of the three samples in Songliao Basin are comparatively high,the average results are 0.282959±0.000026,0.282952±0.000018 and 0.282925±0.000028.εHf(t)average contents are 8.92±0.92,8.47±0.65 and 8.00±10.The model ages of the samples are young,centre at 490-575Ma.The zircon in the Mesozoic rocks have plusεH(ft)results,the model ages of Hf isotope centre at early Paleozoic.This indicates that the source of the rocks is the new crust which growth from the depleted mantle in early Paleozoic,they melted in Mesozoic and formed the rocks.4 The magmatic source of the volcanic rocksThe volcanic rocks are usually used as the probe of deep lithosphere,their Hf isotope can judge the characters in magmatic source.The two kinds of samples in this thesis have identical ¹⁷⁶Hf/¹⁷⁷Hf ratios andεHf(t)results,this reflects that all the volcanic rocks have alike magmatic source,and the difference between the geochemistry elements,especially the Sr,Yb and Sr-SiO₂ diagram indicates the diversity between the rocks is not the results of crystallization differentiation function.Therefore,the formation mechanism of the two kinds of rocks is that they had the same magmatic source,but the depth was different.In early Cretaceous,the rocks in different depth of the crust melted,and formed different types of magma.The adakite rocks with high Sr and low Yb in Shibeiling formed in the lower crust with high pressure,and the I-type rhyolite with low Sr and high Yb in Jiutai formed in the mid crust with low pressure,these magma which came from the different depth erupted into the surface and formed different kinds of rocks.5 The tectonic setting of the volcanic rocksThe volcanic rocks in southeastern Songliao Basin are riched in LREE and LILF,the HFSE are comparatively lo,have the post-collision characters.The early Cretaceous magmatic function in northeastern China behaved the magmatic movement of extension setting,represented by the appearance of bimodal volcanic rocks and A-type granites.In the post-collisional period of orogenics,the thickened lithosphere mantle delaminated for high density,the tectonic system changed from collapse to extension.In this context,asthenospher mantle went up,or the underplating function of basaltic magma heated the lower crust,leaded to the mafic rocks in the lower crust partial melted,these created favorable condition for the adakite and the I-type rhyolite.The study of the geochemical characteristics of the volcanic rocks in southeastern Songliao Basin,combined with the general geology situation,indicated the tectonic setting in southeastern Songliao Basin was post-collisional condition of orogenics.6 The restricting of crustal growth in Xing'anling-Mongolian Orogenic BeltThe Xing'anling-Mongolian Orogenic Belt(XMOB)had large-scale crustal growth in Phanerozoic,base on the analysis of Hf isotope,the dominating growth time was Paleozoic,and the primary function of growth was the mantle substance entered into the crust with the manner of underplating function.The Mesozoic volcanic rocks in Great Xing'an Range had two different magmatic source at least,the enriched mantle source and the young crust source.Because the XMOB had the characters of excessive blocks and evolvement phases,the Hegenshan-Heihe suture zone was an important border of XMOB,the close time of the suture zone was late Paleozoic,and oceanic crust began to subduction from south to north in 330Ma,it reflected that the oceanic crust subduction in late Paleozoic caused the formation of enriched mantle.In the process of subduction and subtraction of oceanic crust,the mantle wedge formed enriched mantle-the source of basalts in Mesozoic-through the metasomatism of oceanic crustal melt or fluid,the mafic magma came from the partial melting of mantle wedge underplated the lower crust and formed the source of granites and acidic volcanic rocks.Therefore,the closure of Paleo-Asian Ocean in late Paleozoic was the significant reason for the formation of the enriched mantle source in Mesozoic in Great Xing'an Range and the large-scale crustal growth in Phanerozoic in the Xing'anling-Mongolian Orogenic Belt.