秦岭显生宙地幔组成及其演化

PHANEROZOIC MANTLE COMPOSITION AND EVOLUTION OF THE QINLING OROGENIC BELT

通过对秦岭造山带及扬子克拉通北缘显生宙时期 3个含地幔捕虏体的煌斑岩、钾镁煌斑岩、碱性玄武岩以及 11个不含捕虏体的辉石岩、辉长岩、玄武岩出露点的岩石地球化学对比研究 ,揭示出研究区地幔演化经历了自古生代的OIB亏损地幔到中生代的高度富集地幔再到中生代末期 -新生代的OIB MORB的亏损地幔的两次明显变更。制约这种变更的主要因素是熔融岩浆时源区发生的层圈相互作用类型。鉴于大陆岩石圈软流层体系的特征 ,有必要划分出岩石圈 /软流层相互作用带(过渡带 ) ,它是大陆岩浆作用的重要源区。

The results of petrological and geochemical study and comparison concerning the samples from 3 exposures of mantle xenolith-bearing lamprophyre, lamproite, alkaline basalt and 11 exposures of Phanerozoic mantle xenolith-free pyroxenite, gabbro, as well as basalts in the Qinling orogenic belt reveal that in Paleozoic the average values of ε(Nd,t) for Langao lamprophyre (413 Ma) and their pyroxenite xenoliths (650 Ma) are 2.32 and 4.43 respectively; for Dahongshan lamproites (326 Ma) and their garnet-lherzolite (502 Ma), which is situated in the north margin of the Yangtze Craton, are -4.7 and (-4.8) respectively; for Lajimiao gabbros (402 Ma) and Wangmuguan-Chunqiumiao gabbros are 2.1 and 2.9 respectively. In Mesozoic after the collision between North China Craton and Yangtze Craton tens of pyroxenite-gabbro intrusions occur along the north margin of north-Dabieshan and the average value of (ε(Nd,t)) is (-13.7). The results also reveal that the mantle evolution have experienced OIB depleted mantle, high enriched crust-mantle zone, and OIB-MORB depleted mantle from Paleozoic, Mesozoic to the end of Mesozoic-Cenozoic. Two tremendous isotopic changes of mantle sources have been shown beneath Qinling orogenic belt. We ascribe the two events to different interaction types of lithosphere-asthenosphere and/or crust-mantle in magma sources. Lamprophyre magma source was controlled by interaction between lithosphere and melts/fluids derived from asthenosphere in Paleozoic. In Mesozoic the interactions in mantle source were more complicated and the end members of interaction may contain uplift asthenosphere, old lithosphere, delaminated lower crust and underplating materials. The interaction between old lithosphere and new uplift asthenosphere was the major process in mantle at the end of Mesozoic-Cenozoic. Along the deep fault, such as Tanlu fault, the uplifting asthenosphere was easy and smooth and the interaction was less strong than that far away from the deep faults. So we suggest that it is necessary to construct the L/A interaction zone or transition zone between the real lithosphere and asthenosphere and it is somewhat similar to the TBL suggested by Mckenzie and the perisphere suggested by Anderson.