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Magma Mixing Genesis of the Jinchang Quartz Diorite in West Qinling Orogen, Western China： Petrographical and Geochronological Constraints and Their Tectonic Implications
详细信息   
摘要
libri’,’sans-serif’; FONT-SIZE: 10.5pt; mso-fareast-font-family: 宋体; mso-fareast-theme-font: minor-fareast; mso-bidi-font-family: ’Times New Roman’; mso-ascii-theme-font: minor-latin; mso-hansi-theme-font: minor-latin; mso-bidi-theme-font: minor-bidi; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA" lang=EN-US>The Jinchang quartz diorite pluton is located in the south part of western Qinling orogen,  western China.  Mafic microgranular enclaves libri; mso-hansi-font-family: Calibri">（libri’,’sans-serif’; FONT-SIZE: 10.5pt; mso-fareast-font-family: 宋体; mso-fareast-theme-font: minor-fareast; mso-bidi-font-family: ’Times New Roman’; mso-ascii-theme-font: minor-latin; mso-hansi-theme-font: minor-latin; mso-bidi-theme-font: minor-bidi; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA" lang=EN-US>MMEslibri; mso-hansi-font-family: Calibri">）libri’,’sans-serif’; FONT-SIZE: 10.5pt; mso-fareast-font-family: 宋体; mso-fareast-theme-font: minor-fareast; mso-bidi-font-family: ’Times New Roman’; mso-ascii-theme-font: minor-latin; mso-hansi-theme-font: minor-latin; mso-bidi-theme-font: minor-bidi; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA" lang=EN-US> are widespread in the pluton,  and are mostly rounded or irregular elongated in morphology.  They commonly have sharp to transitional contact to the host intrusion.  Common presence of needle-like apatite in MMEs,  which,  however,  occurs as short crystals in the host rocks,  suggests mixing of mafic and felsic magmas in the formation of the Jinchang quartz diorite.  Plagioclase in the quartz diorite have oscillatory zoning in terms of An content,  which are 30~40 in the core,  libri; mso-hansi-font-family: Calibri">〉libri’,’sans-serif’; FONT-SIZE: 10.5pt; mso-fareast-font-family: 宋体; mso-fareast-theme-font: minor-fareast; mso-bidi-font-family: ’Times New Roman’; mso-ascii-theme-font: minor-latin; mso-hansi-theme-font: minor-latin; mso-bidi-theme-font: minor-bidi; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA" lang=EN-US>50 in the mantle,  and less than 10 in the rims.  A significant compositional gap was observed between the core and rims.  Inclusions of amphibole and biotite were also found in the rims,  but not in the core.  Amphiboles in host rocks are mostly hornblende and magnesium hornblende,  classified as SiO2-saturated type.  Amphiboles in MMEs are chemically complex,  with both SiO2-saturated magnesium hornblende and SiO2-unsaturated pargasite and pargasitic hornblende.  The Al2O3 and TiO2 contents of hornblende in the MMEs decrease abruptly from the core to the rims,  indicating the core formed in a higher-temperature environment compared to the rims.  Biotite in the host rocks and MMEs are also different.  Biotites in host rocks are either iron or magnesian biotite,  whereas they are exclusively magnesian biotite in MMEs.  In the diagram of ΣFeO/libri; mso-hansi-font-family: Calibri">（libri’,’sans-serif’; FONT-SIZE: 10.5pt; mso-fareast-font-family: 宋体; mso-fareast-theme-font: minor-fareast; mso-bidi-font-family: ’Times New Roman’; mso-ascii-theme-font: minor-latin; mso-hansi-theme-font: minor-latin; mso-bidi-theme-font: minor-bidi; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA" lang=EN-US>ΣFeO libri; mso-hansi-font-family: Calibri">＋libri’,’sans-serif’; FONT-SIZE: 10.5pt; mso-fareast-font-family: 宋体; mso-fareast-theme-font: minor-fareast; mso-bidi-font-family: ’Times New Roman’; mso-ascii-theme-font: minor-latin; mso-hansi-theme-font: minor-latin; mso-bidi-theme-font: minor-bidi; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA" lang=EN-US>MgOlibri; mso-hansi-font-family: Calibri">）libri’,’sans-serif’; FONT-SIZE: 10.5pt; mso-fareast-font-family: 宋体; mso-fareast-theme-font: minor-fareast; mso-bidi-font-family: ’Times New Roman’; mso-ascii-theme-font: minor-latin; mso-hansi-theme-font: minor-latin; mso-bidi-theme-font: minor-bidi; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA" lang=EN-US> vs.  MgO for biotite,  both the host rock and MMEs samples fall into the transitional field between crust and mantle,  with the MMEs samples closer to the mantle field.  Collectively,  field and petrographic data suggest that magma mixing has been important in the formation of the Jinchang quartz diorite.  Laser ablation ICP-MS zircon U-Pb dating yields crystallization ages of 212±2 Ma and 215±1 Ma for the host rocks and MMEs,  respectively,  indicating contemporaneous formation and providing additional evidence supporting magma mixing.  Petrographic evidence and U-Pb ages demonstrate that the Jinchang quartz diorite was emplaced in a post-collisional extensional environment,  which could have facilitated partial melting of enriched lithospheric mantle and lower crust,  and subsequently mixing of mafic and felsic magmas to form the quartz diorite.

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