Petrology, Zircon U-Pb Geochronology, And Petrogenesis Of Early Devonian Yuejinshan Intrusive Complex In The Eastern Kunlun Orogen

Yuejinshan intrusive complex is located in the east part of Eastern Kunlun orogenic belt, the north part of Middle Kunlun Faults and the south part of Qaidam Basin. Through the researches of petrography, zircon U-Pb geochronology, geochemistry, whole rock Sr-Nd isotope and zircon Hf isotope, and combining with the results of regional and field studies, this paper discusses systematically petrogenesis and tectonic environment of Yuejinshan intrusive complex.The important cognition which the author has acquired as follows:1. Yuejinshan intrusive complex is mainly made of four different rocks, which are monzogranite, granodiorite, diorite and gabbro. Based on their contact relationships in the field, we can infer that gabbro intruded firstly, then granodiorite and monzogranite intruded in turn, and dirorite intruded at last;2. The formation time of granodiorite is 407±3 Ma, and those of diorite and gabbro are 405±3 Ma and 406±3 Ma, respectively. The formation times of them are as same as each other in the range of analytical error of zircon LA-ICP-MS U-Pb isotope.3. Yuejinshan monzogranite and granodiorite are characterized by relatively high silicon (the contents of SiO2 are 72.02～73.72% and 69.69～71.02%, respectively),low magnesium (the contents of MgO are 0.30～0.75% and 0.86～1.28%, respectively; The values of Mg# are 32.16～41.59 and 32.16～41.59,respectively),low titanium (the contents of TiO2 are 0.19～0.28% and 0.32～0.43%, respectively) and low calcium (the contents of CaO are 1.43～1.79% and 1.94～2.85%, respectively). Their values of A/CNK are higher than 1.0, but less than 1.1, so they could be slightly prealuminous granites. There is a significantly negative correlation between FeOt, MgO, TiO2, Al2O3, P2O5, MnO and SiO2, which indicates the consanguinity of magma evolution. Diorite and gabbro are characterized by relatively low silicon (the contents of SiO2 are 45.98～51.96% and 55.43～58.68%, respectively),high magnesium (the contents of MgO are 4.60～6.42% and 2.97～3.98%, respectively; The values of Mg# are 47.60～50.26 and 46.55～51.49,respectively),high titanium (the contents of TiO2 are 4.50～6.54% and 0.82～1.11%, respectively) and high calcium (the contents of CaO are 8.98～10.32% and 5.90～7.55%, respectively).4. There are similar characteristics between Yuejinshan monzogranite and granodiorite in their Chondirte normalized REE patterns and primitive mantle normalized trace element spider diagrams, such as:obvious fractionationed REE (the values of LaN/YbN are 6.39～33.71 and 7.65～44.70, respectively; the values of LREE/HREE are 6.21～12.78 and 13.80～26.84, respectively), negative Eu anomalies (the values of 8Eu are 0.42～0.58 and 0.43～0.71, respectively), the enrichment of large ion lithophile elements (LILE) and the depletion of high field strength elements (HFSE). Diorite is characterized by fractionationed REE (the values of LaN/YbN are 9.30～19.64, and the values of LREE/HREE are 7.95～11.22), slightly negative Eu anomalies (the values of 8Eu is 0.83～0.88), relatively steep HREE patterns, the enrichment or slightly depletion of large ion lithophile elements (LILE) and the depletion of high field strength elements (HFSE).The relatively steep HREE pattern(the Y/Yb values are 12.23～12.84) of diorite indicates that there might be some plagioclases and garnets left in the source. Gabbro is characterized by slightly fractionationed REE (the values of LaN/YbN are 1.94～3.15, and the values of LREE/HREE are 2.65～3.48), obvious positive Eu anomalies(the values of 8Eu are 1.17～1.69), relatively flat HREE patterns and the obvious depletion of Nb-Ta-Ti. The relatively flat HREE patterns of gabbro indicates that there are a lot of amphiboles left in the source, and the obvious depletion of Nb-Ta-Ti is in accordance with the occurrence of rutile.5. Monzogranite and granodiorite have the same type of K-fledspar (orthoclase), but the An numbers of granodiorite (An%=21.74～29.23%, oligoclase) are higher than monzogranite (An% =32.56～34.15%, andesine). The type of plagioclases from diorite is mostly andesine-labradorite (An%=47.44～54.88%), while that of plagioclases from gabbro is mostly labradorite (An%=49.11～62.76%). The biotites of the four different rocks in Yuejinshan pluton are magmatic biotites. The biotites of monzogranite and granodiorite are ferrous, while those of diorite and gabbro are magniferous. Compared with monzogranite, the biotites of granodiorite have relatively higher magnesium and titanium, lower iron and aluminium. These indicate that the formation of granodiorite may be different from monzogranite.6. TheⅠsr andεNd(t) values of monzogranite are 0.710～0.740 and -4.05～-5.80 respectively, which are as same as crust materials; Granodiorite has relatively lowⅠSr values (0.710) and highεNd(t) values (-3.54～-3.71), which indicates that its source is similar with monzogranite, but it might be contaminated by some mantle material. This can also confirmed by zircon Hf isotope (theεHf(t) values are -2.05～0.79; two peak values in the histogram of zirconεHf(t) values are-1.93 and 0.51, respectively); TheⅠsr andεNd(t) values of diorite are 0.708～0.709 and -3.33～-3.95,respectively, which indicates its source might be an enriched mantle; TheⅠsr values of gabbro (0.711～0.714) are higher than monzogranite, which indicates it might be contaminated by cust materials. The variation of itsεNd(t) values is great, from -3.44 to -6.82. ItsεNd(t) values are positively correlated with SiO2, but itsⅠsr values negatively related with SiO2..This indicates that gabbro might undergo AFC processes which assimilated crust materials. ZirconεHf(t) values (-2.13～1.11) and two peak values in the histogram (-1.67 and -0.46, respectively) indicate the gabbro might be also derived from an enriched mantle.7. With a comprehensive consideration on many different indexes, such as experimental petrology, whole rock Sr-Nd isotopes, zircon Hf isotope, ages of old ziron captured by granite, mineral chemistry, field studies and so on, we concluded that, Yuejinshan prealuminous granites (include monzogranite and granodiorite) were formed by partial melting of crust materials (meta-greywacke),but the source of granodiorite might be contaminated by some mantle materials, while diorite and gabbro were formed by partial melting of an enriched mantle, and there might be some plagioclases, garnets, Fe-Ti oxides and rutiles in the source of the diorite.8. Yuejinshan prealuminous granites (～407Ma) associated with diorite and gabbro is the typical rock association of post-collisonal settings. In the period of post-collison with lithospheric thinning and asthenosphere upwelling, mantle-derived magmas emplaced into the crust, which provide enough heat for partial melting of crust materials (meta-greywacke) and the production of prealuminous granites; On the other hand, which provide materials, and inject into the crust-derived magma and mix with it, such as the source of granodiorite contaminated by some mantle materials.9. Nd isotopic model ages (T2DM=1-33～1.78Ga) of Yuejinshan prealuminous granites and the other Palaeozoic granites in the Eastern Kunlun suggest that the east Kunlun terrane have a same affinity for Yangtze terrane.