锆石U-Pb定年及Hf同位素测定结果表明,鄂尔多斯盆地周缘的华北板块北缘、兴蒙造山系及扬子板块—秦岭—大别—苏鲁造山带等构造单元系统具有明显不同的形成与演化历史。华北板块北缘锆石年龄平均值为1 837Ma,最强烈的岩浆活动出现于2 200～1 800Ma,该期锆石约占全部的40%;次为强烈的岩浆活动在2 800～2 200Ma,其众数在全部锆石中约占30%;1 500～1 200Ma、500～100Ma这两个阶段形成的锆石在全区所占比例各约为15%。华北板块北缘最突出的特征是基本不含1000～700 Ma期间形成的锆石,〉3 000Ma的锆石在全区分布极为有限。锆石Hf同位素亏损地幔模式年龄表明华北板块北缘平均值为2.55Ga,较U-Pb平均年龄老,说明2 200～1 800Ma期间形成的锆石含有较多的古老地壳再循环组分。Hf亏损地幔模式年龄最强峰值约为2.8Ga,与全岩Nd亏损地幔模式年龄的峰值相一致,Hf模式年龄为3.0～2.25Ga的颗粒占全部的近95%,证明华北板块北缘的地壳增生主要在太古宙至古元古代期间。Hf同位素亏损地幔模式年龄〉3.0Ga的锆石颗粒所占比例不到0.1%,另外近5%锆石的模式年龄分布于中元古代。晚古生代—中生代所形成的锆石均是先存地壳组分,尤其是中元古代增生地壳的熔融作用形成。兴蒙造山系中锆石U-Pb年龄平均值为497Ma,最强峰分布于石炭纪（约320Ma）,石炭纪—二叠纪末（350～250Ma）形成的锆石所占比例达30%以上。新元古代至早古生代（600～440Ma）形成的锆石占全部锆石的55%以上,而中元古代末—新元古代期间（1 200～600Ma）形成的锆石在全区仅占4%。中元古代以前形成的锆石非常有限,说明该区最早形成的地壳组分在兴蒙造山系的形成过程中较充分地参与到后期的岩浆作用过程中。兴蒙造山系中锆石相应的Hf同位素亏损地幔模式年龄平均为1.13Ga,明显较相应的U-Pb年龄老,最强峰值出现于约0.6Ga。Hf亏损地幔模式年龄为0.7～0.28Ga的颗粒在兴蒙造山系所占比例达57%,证明该区最强烈的地壳增生发生于新元古代至古生代期间。Hf同位素亏损地幔模式年龄分布于1.5～0.7Ga的锆石在全区约占38%,说明此期间也是该区地壳较强烈的增生期。Hf亏损地幔模式年龄大于1.5Ga的锆石所占比例不到5%,古生代以后兴蒙造山系也基本没有明显的地壳增生。扬子与秦岭—大别—苏鲁造山带构造单元中的锆石U-Pb年龄平均为799Ma,年龄为1 300～750Ma的锆石在全部锆石中约占70%。晚古生代—燕山期形成的锆石约占20%。年龄在3500～2 650Ma、2 118～1 680Ma的锆石在该区各约占5%。结合扬子与秦岭—大别—苏鲁造山带平均为1.56Ga的Nd亏损地幔模式年龄特征,说明1 300～750Ma期间该区较强烈的岩浆作用事件中有较多的古老地壳组分加入其中。锆石U-Pb年龄及Hf同位素组成均说明鄂尔多斯盆地周缘各构造单元具有不同的形成演化历史。地壳是幕式增长的,但各构造单元每幕发生的时间、强度存在明显差别。因此,由盆地中不同时代地层中碎屑锆石U-Pb年龄及Hf同位素组成及全岩Nd同位素特征的系统研究可反演盆地物源供给与周围构造单元之间的关系。%U-Pb dating and Hf isotopic composition of zircons from the northern margin of North China Platform,the Great Hingan-Mongolian Orogen,and the Yangtze Platform-Qinling-Dabie-Sulu Orogen around the Ordos Basin show that its surrounding tectonic units have different histories of formation and evolution.Average age is 1837 Ma based on U-Pb dating of 885 zircon grains from the northern margin of North China Platform.The most intensive magmatism peak occurred between 2200 and 1800 Ma;the zircon grains within this range amount to 40% of the total.The secondary intensive period is between 2800 and 2200 Ma;percentage of zircon grains in this range is ~30%.The zircons crystallized in 1500-1200 Ma and 500-100 Ma occupy ~15% respectively.The northern margin of North China Platform is characterized by a lack of 1000-700 Ma zircons.The zircons older than 3000 Ma are very rare.The averaged Hf isotopic depletion mantle model age of zircons is 2.55 Ga,which is older than that of U-Pb ages,suggesting that zircons crystallized between 2200 and 1800 Ma contain older re-cycled precursor crustal components.The most intensive peak of Hf isotopic depletion model age is ~2.8 Ga,consistent with that of Nd isotopic depletion mantle model ages.The zircon grains with Hf model ages between 3.0 and 2.25 Ga account for ~95%,which implies that the crustal accretion from the depleted mantle took place between the Archean and the Paleoproterozoic.Other ~5% zircon grains were formed by Mesoproterozoic crustal accretion event.The Late Paleozoic-Mesozoic magmatism was predominately controlled by anatexis of precursor crustal components,especially the Mesoproterozoic juvenile crust.The percentage of zircon grains with the Hf model ages older than 3.0 Ga is less than 0.1%.The average U-Pb age of zircons in the Great Hingan-Mongolian Orogen is 497 Ma;the most intensive peak is around the Carboniferous,i.e.~320 Ma.Carboniferous-Permian zircons account for over 30% in the orogen.Another population of zircon U-Pb ages is around the Neoproteozoic-Early Paleozoic,i.e.600-440 Ma,whose percentage is 55% of the total.The percentage of Late Mesoproterozoic-Neoproterozoic（1200-600 Ma） zircons is ~4%.The corresponding average Hf isotopic depletion model age of the zircons in the orogen is 1.13 Ga,and the most intensive peak is around ~0.6 Ga.The Hf model ages of zircons dominantly distribute between 0.7 and 0.28 Ga;the zircons within this range account for 57% of the total,which implies the most intensive accretion period of the orogenic crust from the depleted mantle.Other intensive accretion episode is between the Mesoproterozoic and the Neoproterozoic（1.5-0.7 Ga）;the percentage of zircons in this range is up to ~38%.The zircons with Hf model age between 3.3 and 1.5 Ga is less than 5%.The orogenic crust almost has no juvenile component from the depleted mantle since the Paleozoic.The average U-Pb age of zircons in the Yangtze Platform-Qinling-Dabie-Sulu Orogen is 799 Ma.The zircon population between 1300 and 750 Ma is ~70% of the total.The other intensive magmatism period is between the Late Paleozoic and the Yanshanian;the zircons formed in this period account for ~20%.Paleoarchean-Paleoproterozoic（3500-2650 Ma） and Paleoproterozoic-Mesoproterozoic（2118-1680 Ma） zircons account for ~5%,respectively.On the basis of averaged Nd isotopic depletion mantle model age of 1.56 Ga,it is suggested that much old precursor crustal components were involved in the magmatism event between 1300 and 750 Ma.The tectonic units around the Ordos Basin have different histories of formation and evolution.Their crustal growth has been episodic.But the age and intensity of each episode show significant differences among the tectonic units.Thus,the strata of the basin may decipher corresponding provenances through U-Pb dating and Hf isotopic composition of detrital zircons in them.
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