Petrogeochemistry,Zircon U-Pb Geochronology And Tectonic Significance Of The Songlinkou Granite In The Eastern Margin Of The Songpan-Ganzi Orogenic Belt

The Songlinkou pluton,in the eastern of the Songpan-Ganzi orogenic belt and adjoins the Xianshuihe fault zone,consists the marginal granodiorite and the central monzonitic granite,with abundant monzodioritic enclaves.The content of enclaves in the margin are more than the center,the diameter of enclaves are 10³0 cm,and a few of them more than 50cm.In this paper,the petrology,geochemistry,zircon U-Pb dating and Lu-Hf isotope studies of 11 samples were carried out.Through this study,the following main results have been achieved.The pluton and enclaves are rich in hornblende,biotite and other minerals,in addition,enclaves contain pyroxene minerals.The content of SiO₂ and total alkali in enclaves are 54.24%,7.34%,K₂O/Na₂O is 4.24,and??Rittmann index?=4.63.Respectively,the SiO₂ content of the intruded rock is between 56.56%and 68.99%,the total alkali content is 3.78%to 6.82%,and the K₂O/Na₂O is 1.02¹.93,??=1.01¹.93?,the pluton belongs to the high potassium calc-alkaline rock.The central monzonitic granite is weak peraluminous?aluminum saturation index A/CNK=0.736⁰.958?,and the marginal granodiorite is metaluminous?aluminum saturation index A/CNK=0.736⁰.958?.the rock has relative high Mg#values?Mg^#=46.73⁶1.27?,the P₂O₅ is negative correlation with the SiO₂ content.The pluton has the characteristics of enrichment of light rare earths,the LREE/HREE=2.67⁸.31,La_N/Yb_N=2.11⁹.74,the difference of light and heavy rare earth elements are obvious,the patterns of LREE are right-leaning,and HREE are flat curve.All the rock samples have strong Eu negative anomaly,the enclaves and the granodiorite with weak negative anomaly of Ce,however the monzonitic granite with slight Ce-positive anomaly.Songlinkou pluton is rich in LILE,The average Rb/Sr is 0.44,and HFSE of Ta?Nb?P?Ti are depleted.,The separation and crystallization of amphibole and biotite may be related to the Nb/Ta ratio,and the strong depleted of P and Ti may be related to the crystallization and separation of apatite and titanium-containing minerals such as ilmenite.In both the Zr-SiO₂ and the Y-SiO₂ diagrams,the samples are located in the I-type granite area.From the characteristics of mineral composition,major elements and trace elements,the Songlinkou pluton belongs to high potassium calc-alkaline I-type granite.The zircon U-Pb results of the granodiorite and monzogranite and enclaves are212.6±1.0 Ma?MSWD=0.55?,222.4±1.1 Ma?MSWD=0.39?,225.2±1.0 Ma?MSWD=0.35?respectively.The difference of crystallization ages between granodiorite and monzograniteusions are about 10Ma.They were formed by twice intrude which came from the same source rock.The ?_Hf?t?of monzogranite is-9.09 to-6.86 and enclaves are-5.27 to-1.09,respectively.The T^C_DM of monzogranite is between 1524.9 Ma and 1666 Ma,and enclaves are between 1326.7Ma and 1592.8Ma.In the zircon U-Pb age-?_Hf?t?diagram,the samples fall above the 1.87Ga crustal evolution line.The Th/U and Nb/Ta ratios of monzogranite is 5.01 to 12.21 and monzodiorite enclaves are 2.62 to 10.0,respectively,and it is suggesting that the Songliankou pluton is the partial melting of the lower crust.Under the compressional tectonic background of arc-land collision and land-land collision,the asthenosphere material in the base of Songpan-Ganzi orogenic belt upwelled,and the addition of mantle-derived material resulted in partial melting of the middle Proterozoic Yangtze craton basement in the lower crust of the orogenic belt.The mixing of basic magma and crust melting magma is the main mechanism for the formation of enclaves in Songlinkou pluton.With the continuation of orogeny and the thickening of the crust,lithosphere delamination occurs in the lower crust,and a large number of mantle-derived magma upwelling due to the transformation from extrusion to extensional environment.As a result,the melting degree of the crust is increased,and multi-level melting occurs within crust.The granodiorite is the result of magmatic mixing,and the mixing of crust-mantle is more thoroughly,and the geochemistry data affinity of enclaves and granodiorite is better than that of earlier monzonitic.The later granodiorite carries more mafic enclaves.