Raobazhai ultramafic rock(RUR)consists mainly of harzburghe.Their two distinctcharacteristics are:(i)mylonitization of harzburgite is quite strong;(ii)metasomatism of mantle is developed.Trace element,REE,Nd-and Sr-is...Raobazhai ultramafic rock(RUR)consists mainly of harzburghe.Their two distinctcharacteristics are:(i)mylonitization of harzburgite is quite strong;(ii)metasomatism of mantle is developed.Trace element,REE,Nd-and Sr-isotope geochemistry show that the RUR is much similar to that ofcontinental lithospheric mantle(CLM)rather than oceanic lithospheric mantle(OLM).Melt composition cal-culated by cumulate demonstrates that the characteristics of island are basalt,which implies that the source ofmantle flow might be related to the subduction process.It is possible that RUR represents a remmantof CLM above the Proterozoic subduction zone.展开更多
The ultramafic massif at Raobazhai in North Dabie is located in the suture zone between the Yangtze craton and North China craton. The Re-Os isotope compositions of the massif are used to decipher the origin and tecto...The ultramafic massif at Raobazhai in North Dabie is located in the suture zone between the Yangtze craton and North China craton. The Re-Os isotope compositions of the massif are used to decipher the origin and tectonics of the ultramafic rocks involved in continental subduction and exhumation. Fifteen samples were collected from five drill holes along the main SE-NW axis of the Raobazhai massif. Major and trace element compositions of the samples show linear correlations between MgO, Yb and Al2O3. This suggests that the massif experienced partial melting with variable degrees and is from fertile to deplete in basaltic compositions. Nine selected samples were analyzed for Re-Os isotope compositions. Re contents range from 0.004 to 0.376 ng/g, Os contents from 0.695 to 3.761 ng/g, ^187Re/^188Os ratios from 0.022 to 2.564 and ^187Os/^188Os ratios from 0.1165 to 0.1306. These indicate that the massif is a piece of continental lithospberic mantle with variable depletion. Using the positive correlations between ^187TOs/^188Os and Yb and Al2O3 respectively, a proxy isochron age of 1.8±0.1 Ga is obtained for the Raobazhai ultramafic massif, which is interpreted to represent a fragment of the ancient subcontinental lithospheric mantle. During Triassic subduction of the Yangtze craton beneath the North China craton, part of the wedge of subcontinental lithospheric mantle was intruded into the subduction belt, and then exhumed to crustal level together with the subducted crustal plate after ultrahigh pressure metamorphism at mantle depths. This ancient lithospheric mantle is now exposed as an orogenic peridotite massif.展开更多
王希斌等根据铙钹寨岩体恢复的原岩由亏损强烈的方辉橄榄岩、纯橄岩和弱亏损的二辉橄榄岩组成以及有豆荚状铬铁矿存在,认为铙钹寨岩体属于大洋岩石圈地幔,是蛇绿岩的成员;并且还根据岩体存在两种地幔橄榄岩组合进一步推断铙钹寨岩体&qu...王希斌等根据铙钹寨岩体恢复的原岩由亏损强烈的方辉橄榄岩、纯橄岩和弱亏损的二辉橄榄岩组成以及有豆荚状铬铁矿存在,认为铙钹寨岩体属于大洋岩石圈地幔,是蛇绿岩的成员;并且还根据岩体存在两种地幔橄榄岩组合进一步推断铙钹寨岩体"可能经历了洋内扩张(形成 MOR 型的地幔残余)和洋内俯冲两个阶段的演化过程"。我们认为,铙钹寨岩体是交代的地幔橄榄岩,它不大可能是蛇绿岩;铙钹寨岩体的特征比较接近大陆岩石圈地慢而非大洋岩石圈地幔;铬铁矿不是判别蛇绿岩的标志;不能根据岩体存在强烈亏损和弱亏损的两种橄榄岩而推断其形成于两种环境。展开更多
基金the National Natural Science Foundation of China
文摘Raobazhai ultramafic rock(RUR)consists mainly of harzburghe.Their two distinctcharacteristics are:(i)mylonitization of harzburgite is quite strong;(ii)metasomatism of mantle is developed.Trace element,REE,Nd-and Sr-isotope geochemistry show that the RUR is much similar to that ofcontinental lithospheric mantle(CLM)rather than oceanic lithospheric mantle(OLM).Melt composition cal-culated by cumulate demonstrates that the characteristics of island are basalt,which implies that the source ofmantle flow might be related to the subduction process.It is possible that RUR represents a remmantof CLM above the Proterozoic subduction zone.
文摘The ultramafic massif at Raobazhai in North Dabie is located in the suture zone between the Yangtze craton and North China craton. The Re-Os isotope compositions of the massif are used to decipher the origin and tectonics of the ultramafic rocks involved in continental subduction and exhumation. Fifteen samples were collected from five drill holes along the main SE-NW axis of the Raobazhai massif. Major and trace element compositions of the samples show linear correlations between MgO, Yb and Al2O3. This suggests that the massif experienced partial melting with variable degrees and is from fertile to deplete in basaltic compositions. Nine selected samples were analyzed for Re-Os isotope compositions. Re contents range from 0.004 to 0.376 ng/g, Os contents from 0.695 to 3.761 ng/g, ^187Re/^188Os ratios from 0.022 to 2.564 and ^187Os/^188Os ratios from 0.1165 to 0.1306. These indicate that the massif is a piece of continental lithospberic mantle with variable depletion. Using the positive correlations between ^187TOs/^188Os and Yb and Al2O3 respectively, a proxy isochron age of 1.8±0.1 Ga is obtained for the Raobazhai ultramafic massif, which is interpreted to represent a fragment of the ancient subcontinental lithospheric mantle. During Triassic subduction of the Yangtze craton beneath the North China craton, part of the wedge of subcontinental lithospheric mantle was intruded into the subduction belt, and then exhumed to crustal level together with the subducted crustal plate after ultrahigh pressure metamorphism at mantle depths. This ancient lithospheric mantle is now exposed as an orogenic peridotite massif.
文摘王希斌等根据铙钹寨岩体恢复的原岩由亏损强烈的方辉橄榄岩、纯橄岩和弱亏损的二辉橄榄岩组成以及有豆荚状铬铁矿存在,认为铙钹寨岩体属于大洋岩石圈地幔,是蛇绿岩的成员;并且还根据岩体存在两种地幔橄榄岩组合进一步推断铙钹寨岩体"可能经历了洋内扩张(形成 MOR 型的地幔残余)和洋内俯冲两个阶段的演化过程"。我们认为,铙钹寨岩体是交代的地幔橄榄岩,它不大可能是蛇绿岩;铙钹寨岩体的特征比较接近大陆岩石圈地慢而非大洋岩石圈地幔;铬铁矿不是判别蛇绿岩的标志;不能根据岩体存在强烈亏损和弱亏损的两种橄榄岩而推断其形成于两种环境。
