| Hongshanzi basin is located at Hongshanzi County in south Keshiketengqi, Chifeng,south Great Hingan Range, mainly influenced by EW Xar Moron River and NNE GreatHingan Range deep faults. Volcanic rocks in Late Mesozoic widely develop in the district,and the volcanic rock series at the basin are characterized by rock assemblage of alkalineand high-K calc-alkaline rhyolites, NE distributed, with brownish yellow and grayish greensandstone, and tuffaceous shale in the bottom. According to the rock assemblage features,and combining the age data of volcanic rocks obtained in this paper, this rock assemblageshould belong to Xinmin Formation.SHRIMP zircon U-Pb ages show: the ages of two alkaline rhyolites are154.7Ma±1.7Ma, MSWD=0.77and156.5Ma±1.4Ma, MSWD=1.16; the age of one high-Kcalc-alkaline rhyolites is156.5Ma±1.6Ma，MSWD=1.11, suggesting the SHRIMP zirconU-Pb age of volcanic rocks at Hongshanzi basin is155Ma157Ma. And according to theinternational stratigraphic chart, the boundary line of Middle Jurassic and Early Cretaceousis161.2Ma±4.0Ma, and the boundary line of Late Jurassic and Early Cretaceous is145.5Ma±4.0Ma, indicating the alkaline rhyolites at Hongshanzi basin are the product ofvolcanic activities in late Early Jurassic.SiO2content of alkaline rhyolites at Hongshanzi basin is high（70.44%73.19%,averaging at71.95%）, rich in alkaline（Na2O+K2O=9.9%10.78%, averaging at10.33%）,showing the characteristics of rich in K （K2O/Na2O=1.2827.37, averaging at10.17）, butlack of Mg（MgO content is0.11%0.31%, averaging at0.19％） and Ca（CaO content is0.02%0.30%, averaging at0.12％）, plotting within the range of alkaline series rhyolitesboth in the TAS and Zr/TiO2-Nb/Y diagrams. In the Q-A-P diagram, all the samples fallinto the area of alkaline rhyolites, with high D.I between94.0195.75（averaging at94.66）,reflecting high degree differentiation evolution during the rock forming process. The totalamount of REE is relatively high, ΣREE=289.7×10-6415.73×10-6, showing rich LREE obviously, ΣLREE/ΣHREE=7.8310.33（averaging at8.87）; in the chondrite normalizedREE patterns, showing distinct right deviated, incompatible elements such as Rb, Zr andTh are enriched, while Ti, Ba, Sr and Co are depleted, and Eu has strong negative anomaly;in trace element spidergram, the lithophile elements and high field strength elements enrichsignificantly, consistent with low Ba-Sr rhyolites（or low Ti rhyolites, low Sr rhyolites andII type rhyolites） in Great Hingan Range; in Nb、Y、Zr,Ce-10000Ga/Al（,K2O+Na2O）/CaO-Zr+Nb+Ce+Y and FeOT/MgO-Zr+Nb+Ce+Y diagrams, showing the ingredient features ofA type granites; Rb/Sr, Ti/Y and Ti/Zr are0.3734.87（averaging at11.62）,15.0054.83and1.393.19, which is>0.5,<100and <20, respectively, belonging to the range of crustalsourced magma; Rb/Y, Rb/Sr and other12groups of elements’ ratios are approximate withthose in the upper crust, suggesting the provenance are closely related with the upper crust;fluid sensitive elements ratios such as Ce/Yb-Pb/Ce and （La/Sm）N-（Ba/La）Ndiagrams showthe provenance are altered by mantle fluids; while Y/Nb-Yb/Ta and Y/Nb-Ce/Nb diagramsimply that the provenance, together with Ba-Sr rhyolites at Great Hingan Range, areinfluenced by alkaline parent rock from mantle.Alkaline rhyolites at Hongshanzi basin plot in the anorogenic area related within-plate rift in the R1-R2, SiO2-Al2O3, MgO-FeOT, SiO2-FeOT/（FeOT+MgO）, CaO-FeOT+MgO, Nb-Y-Ga and Nb-Y-Ce diagrams; and fall near the transitional area of PA and AA inthe R1-10000Ga/Al diagram, indicating the conclusion of orogeny and transfer intoextensional environment.To sum up, alkaline rhyolites at Hongshanzi basin originate from magma of partialmelted upper crust caused by mantle sourced mafic magma intrusion under the in-plateextensional setting in late Early Jurassic, being influenced by mantle fluids andexperienced high degree differentiation evolution. |
PDF Full Text Request