The Yanshannian Intrusive Rocks And Their Relationship With Metallogenesis In The Chizhou Area,Lower Yangtze River Belt

The intensive and multistage diagenesis and mineralization occurred in the LowerYangtze River during late Mesozoic,which produced large amounts of igneous rocks and polymetallic deposits.However,the Lower Yangtze River Metallogenic Belt(LYRMB)and eastern Jiangnan orogen,which are two tectonic units of the Lower Yangtze area,show obviously different metallogenic features.The Yanshaninan magmatism mainly procdued Cu-Au-Fe deposits in the LYRMB,while a large amount of W-Mo deposits developed in the easetern Jiangnan orogen.Moreover,compared with the first-stage igneous rocks closely related to mineralization,the petrogenesis and tectonic setting of the late-stage A-type granites in the LYRMB are poorly studied and controvercial.The Yanshanian magmatism in the Chizhou area,which is located at the junction of two tectonic units,not only produced Cu-Au deposits,but also Mo-Cu(W)deposits,which is different from the the metallogenic regularity of the two tectonic units.Besides,there are many multi-lithology A-type granites,such as the Huayuangong pluton,in the Chizhou area.Therefore,conducting petrology,mineralogy,whole-rock geochemistry,Sr-Nd-Pb,zircon U-Th-Pb-Hf isotopes and trace element research on the two-stage igneous rocks in the Chizhou area could not only explore their petrogenesis and geodynamic setting,but also facilitate understand the diagenesis and mineralization rules of the Yanshanian epoch in the Lower Yangtze belt.First of all,several early-stage ore-bearing intrusions were choosed as the research object.It was found that the ore-bearing intrusions in the Chizhou area(150?141 Ma)include high-K calc-alkaline series pyroxene diorite,quartz diorite(porphyry)and granodiorite(porphyry),belonging to the products of the first-stage magmatism in the LYRMB in the late Mesozoic.Among them,the Xiaodingchong pyroxene diorite shows low Si O₂ and high Mg O contents,arc-like trace element compositions and enriched Sr-Nd-Pb-Hf isotopic compositions,suggesting that it was derived from partial melting of enriched lithospheric mantle.However,the Pailou granodiorite porphyry shows weakly enriched Sr-Nd-Hf isotopic compositions and high radiogenic Pb isotope,and adakitic rock geochemical features,suggesting that it was derived from partial melting of the Meso-Neoproterozoic thickened accretionary arc crust.The remaining intermediate-acid ore-bearing intrusions could be divided into two groups.One group is derived from fractional crystallization of mantle-derived magma and from partial melting of the Meso-Neoproterozoic thickened accretionary arc crust,whereas the other is produced by mixing of mantle-derived magmas and melts derived from partial melting of the Meso-Neoproterozoic thickened accretionary arc crust.Therefore,the source of the Cu deposits in the Chizhou area was controlled by mantle-derived magmas,whereas the source of the Mo(W)deposits was influenced by the Meso-Neoproterozoic accretionary crust.The age of the first-stage igneous rocks in the LYRMB is getting younger from west to east.Besides,low Ti-in-Zircon temperatures(T_Ti-in-Zrn)and containing large amounts of inherited zircons suggest that the ore-bearing intrusions in the Chizhou area were produced under low temperature and water-enriched conditions.This is consistent with the tectonic setting that the Paleo-Pacific slab subducted into this area at a low angle,forming a metasomtised enriched mantle.Secondly,the Bashan complex,which is located in the central part of the Chizhou andgains little attention,is selected as the representative rock of the late-stage igneous rock to conduct a more detailed research.The Bashan complex consists of one M-type granitoid intrusion(quartz monzonite)and three A-type granitic intrusions,the latter including quartz syenite,syenogranite and alkali-feldspar granite.Zircon U-Pb geochronology shows that these intrusions formed at 126?123 Ma,belonging to the products of the late-stage magmatism in the study area.The quartz monzonites have intermediate Si O₂(60.5–63.1 wt%)and high Na₂O+K₂O(8.66–9.83 wt%)contents,arc-like trace element compositions,enriched whole-rock Sr,Nd and zircon Hf isotopes((⁸⁷Sr/⁸⁶Sr)_i=0.7082–0.7091;?_Nd(t)=-6.9 to-7.1;?_Hf(t)=-5.3 to-8.2)and high radiogenic Pb isotopes(²⁰⁶Pb/²⁰⁴Pb(t)=18.581–18.792).They are inferred to have been derived from partial melting of an enriched lithospheric mantle source,followed by fractional crystallisation and limited crustal contamination.The quartz syenites show high Si O₂(65.9–69.8 wt%)and Na₂O+K₂O(11.3–12.3 wt%)contents,low Mg O(0.14–0.23wt%)contents,high 10⁴*Ga/Al(2.34–3.61)values,arc-like trace element compositions,and whole-rock Nd and Pb and zircon Hf isotopic compositions(?_Nd(t)=-6.8 to-7.0;?_Hf(t)=-7.3 to-12.3;²⁰⁶Pb/²⁰⁴Pb(t)=18.559–18.970)similar to the quartz monzonite,suggesting that they were produced by fractional crystallisation of the quartz monzonite,with some crustal assimilation.The quartz monzonite has higher Ti-in-zircon temperatures(T_Ti-in-Zrn=696–832°C)and lower zircon saturation temperatures(T_Zr=772–818°C)and oxygen fugacities(?FMQ=+1.8 to+2.8)than the quartz syenite(T_Ti-in-Zrn=623–805°C;T_Zr=856–909°C;?FMQ=+3.5 to+4.8),indicating that the zircons from the quartz syenite crystallised at lower temperatures,and that the oxygen fugacity significantly elevated as the magmatic temperature decreases.The syenogranites display high Si O₂(74.6–75.5 wt%)contents,high T_Ti-in-Zrn(671–871°C),T_Zr(799–822°C),and low oxygen fugacities(?FMQ=+0.9 to+3.7),and thus did not evolve from the quartz syenite.They show high 10⁴*Ga/Al values(2.67–2.95),low Mg O(0.1–0.17 wt.%)contents and(La/Yb)_N(7.60–10.19)ratios,pronounced negative Eu anomalies(Eu/Eu*=0.28–0.38)and enriched Sr,Nd and zircon Hf isotopic compositions(?_Nd(t)=-7.2 to-7.5;?_Hf(t)=-5.1 to-14.0),indicating that they were derived from fluid-absent partial melting of Neoproterozoic calc-alkaline granitoid rocks under low-pressure–high-temperature conditions.The alkali-feldspar granites are characterised by high Si O₂(76.5–78.0 wt%)and Na₂O+K₂O(8.34–9.02 wt%)contents,but low Mg O(0.03–0.08 wt%)contents.They show less enriched Nd isotopes(?_Nd(t)=-5.7),a wide range of zircon?_Hf(t)values(-1.9 to-11.2)and high oxygen fugacities(?FMQ=+2.9 to+4.3).They were produced by mixing of highly-fractionated alkaline basaltic magmas and melts derived from partial melting of Mesoproterozoic crust.The inferred petrogenesis of the Bashan complex magmatic rocks suggests that the A-type granites within the LYRMB have multiple magmatic sources and crystallised under a wide range of temperatures,water contents and oxygen fugacities.During the Mesozoic,subduction and rollback of the Palaeo-Pacific slab induced crustal extension and intense crust–mantle interaction,which played a dominant role in forming the A-type granites within the LYRMB.This paper also conducted garnet U-Pb dating to restrict the formation age of theXuqiao-Wugudun skarn Pb-Zn-Ag deposit located in the northeast of the Chihzou area,and to reorganizes the regional diagenesis and mineralization rules.Detailed field geological survey and zircon U-Pb dating revealed that there are two types of dikes in the Wugudun mining.One of dikes is diorite porphyry and formed at 146–141 Ma,closely related to Cu and Mo mineralization;the other is granite and formed at 113.4±2 Ma,closely related to Mo mineralization.The U-Pb dating of garnets in skarn yields ages of148–143 Ma,which are similar to that of the diorite porphyry.Therefore,there are three stages of diagenesis and mineralization in the Chizhou:(1)the first-stage(150–135 Ma)magmatism mainly formed intermediate-acid intrusions and related Cu,Au,Mo,Pb,and Zn deposits;(2)the second-stage(134–124 Ma)mainly produced syenogranite,with small-scale Pb,Zn and Mo mineralization;(3)the third-stage magmatic activity(115–110 Ma)was nearing completion and mainly produced granite dikes,with weak Mo mineralization.Besides,the mineralizing rule in the Chizhou is not the same as that in the LYRMB,and the multistage Mo(W)mineralization is possibly related to multistage reworking of the Meso-Neoproterozoic accretionary curst.This study shows that although the two major stages of magmatic rocks in the Chizhouhave different petrogenesis,they are all related to the westard subduction of the Paleo-Pacific plate.In the late Mesozoic,the Paleo-Pacific slab subducted into the LYRMB at a low angle.During this,dehydration and melting occurred,and the released fluids and melts metasomatised the overlying lithospheric mantle.Continuous dehydration and melting made the subducting slab become denser,and then sink and rollback.At same time,the asthenospheric mantle laterally flowed,heating the metasomatised mantle and resulting in the first-stage magmatism from west to east.The mantle-derived magma intruded to crust and then interacted with different basement rocks,forming the first-stage igneous rocks and various types of metal deposits.Due to the change of the subduction direction and continuous slab rollback,stronger crustal extension occurred in the LYRMB,which caused the mantle-derived magma to rise rapidly to shallow crust,not only forming the second-stage volcanic rocks,but also resulting in melting of shallow crust and in forming the third-stage A-type granite.Therefore,the multi-stage diagenesis and mineralization in the Chizhou reflects the evolution processes from broad continental margina arc to back arc under the setting of the subduction of the Paleo-Pacific plate.