A Study On Geochemistry Of The Granitic Rocks In FuPing Region Of ShanXi Pronvince During Early Triassic And Its Geological Significance

A quantity of granitic rocks had been generated as a result of collision of the QinLing and YangZi plate during early TriassicParticularly.the WuLong pluton located in FuPing region had accounted for 85% of NingShan rocks.As we known,the origin of the rocks had played an important role in the formation of QinLing-DaBie,specific in the process of dynamics and evolution.Therefore,making a further study to this field will trace effectively the geodynamic mechanism and the formation of QinLing.The data of occurrence, petrography, geochemistry and zircon U-Pb chronology are reported,the source and geololgical significance of the pluton are discussed in this paper.The conclusion indicates:1 The Zircon LA-ICP-MS dating for the Xichahe pluton yields 213.6±2.2Ma.In addition,the dating for the WuLong pluton is 225.3±6Ma and 208.5±2.2Ma slight later the orogenic stage of QinLing .2 Lithologically,the Xichahe pluton consists mainly of quartz dioritic which is closer to the adakites in geochemistry.Its geochemistry is with SiO₂=56.90%⁶4.40% Al₂O₃=14.87%¹8.37%,Na₂O>K₂O,Mg^# (52.9⁶6.4)>47;and enriched in LILE and LREE,Sr>800μg/g,high Sr/Y ration (44.9⁸8.1), silghtly negative Eu anormaly (8Eu=0.65¹.10);flatter depleted in HREE and Y (Y=10.3μg/g²2.7μg/g, Yb=0.79μg/g².15μg/g,Y/Yb= 10.3²2.7) ,obivious differenation((La/Yb)_N=6.7³0.0) between.LREE and HREE,high enriched in Cr, Co and Ni.3 The WuLong pluton is mainly composed of damellite and spotted biotite granite Damellite are accompanyed by a few of mafic microgranular enclaves which indicates the mixing and mingling process of two kinds of magmas.As the Xichahe pluton,the WoLong pluton shows the partial characteristics of adakites as SiO₂=65.29%⁷1.30%,Al₂O₃=13.96%¹9.12%;Sr>650μg/g,high Sr/Y ration (57.8¹60.0), depleted in HREE (Y=4.76μg/g¹1.1μg/g, Yb=0.33μg/g⁰.96μg/g),slightly negative Eu anormaly(δEu=0.70⁰.82);In the trace element patten,rocks suggest that the depletion of high field strength element and the enrichment of LILE.The geochemistry of mafic microgranular enclaves is obviously different from the host rocks which is with lower SiO₂(52.86%⁵5.76%),σ=3.1,Na₂O+K₂O=5.0%⁷.1 %,high enrichment in LILE and LREE.In the diagram of Hark ,it indicates the different variation and obvoius content discontinuity.Otherwise,it suggests a certain magmas mixing trend in the NaO/CaO-Al₂O₃/CaO.These characteristic show the mafic microgranular enclaves represented a mantle-sourcedmagma.4Two plutons had a simlar composition in isotopic and it could be assumed that they had the same protolith.The data of isotopic is (I43Nd/l44Nd );=0.51246-0.51235,eNd=-1.53—3 .26,TDm= 1 ? 16Ga 1.38Ga, ("'Sr/^Sr) j=0.70543^O.70645;the WuLong pluton is (143Nd/144Nd),=051223⁰.51225,eNd=-2.26^-2.66> Tdm-1 .01Ga¹.02Ga, (87Sr/86Sr)i=0.70445⁰.70503.The isotopic characteristic are close to some of the metabasalt in Yaolinghe group.lt suggested its rocks sources maybe have an intrgreated relation with the underplation of QinLing continent in l.lGa.The source of rocks has three partion,It is the lower continent crust deriving from the mantle in l.lGa,the primitive basic magma of the mafic microgranular enclaves and the ancinent continent crust.5 Combined with previous studies of QinLing belt,we confer the maganism of the WuLong and the Xichahe pluton that the breakoff oceanic crust generated the upwelling asthenosphere as the ocean basin subduction towards the North. The upwelling asthenosphere triggered the more partiton of the thicked mafic lower continent .Hence,such maganism generated the WuLong and Xichahe pluton which had a similar geochemistical characteristics with adakite rocks.