略谈藏东吉塘群

[摘 要]以往的研究工作将藏东吉塘群分两个组:上部以片岩、变质砂砾岩为主的地层名酉西组;下部以片岩、片麻岩、混合岩为主的地层名恩达组,时代均属前震旦纪。近几年,开展1:25 万编图项目的研究,恩达组可细分为两个岩性段:混合岩段和以片岩、片麻岩、变粒岩、斜长角闪岩及大理岩共同组成的深变质岩段。酉西组中获得新的同位素年龄:240±12Ma( 锆石U-Pb)、226±2Ma( 白云母Ar-Ar),时代可能为古生代,更名为酉西岩群。恩达组的混合岩中新获得锆石SHRIMP U-Pb 同位素年龄254±8Ma、232±9Ma、227±1.8Ma,原1:20 万类乌齐、拉多幅曾于相当恩达组的花岗片麻岩中获得锆石U-Pb年龄253±9Ma、195Ma,这一岩段的混合岩可能是华力西至燕山期变形变质的花岗岩侵入体,应从吉塘群中分离出去。原恩达组中除混合岩外的地层更名为吉塘岩群,时代属古中元古代。     

还原性流体与斑岩型矿床成矿机制探讨

目前已经广泛认同斑岩型Cu-Mo-Au矿床是在氧化性相对较高的含矿流体作用下形成的。但是随着研究的深入,逐渐发现了一系列具备还原性特征的斑岩型矿床,这些矿床往往不发育表征高氧逸度的原生磁铁矿和硫酸盐矿物。本文针对目前发现的与还原性I型花岗岩相关的斑岩矿床进行了综合分析,发现此类矿床的成矿流体中普遍含有CH4等还原性流体成分,并且伴生有CO2。结合相关资料分析认为还原性流体中所含有的CH4可能来自邻近的S型花岗岩的混染作用,但也不排除是经地球排气作用从地幔进入到地壳的可能性。结合典型还原性斑岩型矿床研究,给出了一个化学模型,认为CH4和SO2属于岩浆系统自生成分,在特定化学物理阶段发生反应,形成H2S和CO2,从而抑制了硫酸盐矿物的形成。反应所生成的H2S可以与Mo结合形成辉钼矿矿化,而Cu-Au由于在氧逸度较低的环境下具备更高的活性,易于富集在气相流体中,所以可以迁移到某一距斑岩系统较远的有利部位沉淀成矿,从而形成以Mo矿化为核心区,外围Cu-Au矿化的模式。     

金鸡岭产铀花岗岩体印支期侵位的岩浆动力学证据及其构造意义

通过对南岭西段金鸡岭花岗岩体地质-岩石地球化学特征研究,判明该岩体的侵位深度(7.5km)、围岩温度(270℃)及岩浆初始温度(950℃),建立起金鸡岭花岗岩体的数学计算模型,分别计算得出:金鸡岭花岗岩熔体侵位后,其初始温度降低至结晶温度所需的时间(Δtcol)为3.91Ma;由于结晶潜热释放而使结晶过程延长的时间(ΔtL)为2.92Ma;由于金鸡岭花岗岩体放射性元素含量(U——16.5×10-6,Th——51.3×10-6,K2O——4.82%)是世界平均花岗岩放射性元素含量(U——5×10-6,Th——20×10-6,K2O——2.66%)的3倍左右,金鸡岭花岗岩熔体侵位后产生的放射性成因热使结晶过程延长的时间(ΔtA)为34.5Ma,远长于按世界花岗岩平均放射性元素含量计算的ΔtA*(2.82Ma)。金鸡岭花岗岩体的侵位-结晶时差(ΔtECTD)为41.3Ma,结合锆石U-Pb年龄值(156Ma),通过反演计算得出金鸡岭花岗岩体侵位年龄值(tE)为197.3Ma,从而为该岩体属于印支期侵位提供了重要的岩浆动力学证据。     

长江中下游金牛盆地花岗斑岩和流纹斑岩的锆石U-Pb年龄、Hf同位素组成及其地质意义

金牛盆地位于鄂东南矿集区,是长江中下游成矿带重要的火山岩盆地之一.相对宁芜和庐枞盆地,金牛盆地找矿一直没有取得重大突破,对其中次火山岩的年代学、成因和成矿属性的研究也很少.本次选择盆地中心吴佰浩地区深部钻孔中的花岗斑岩和流纹斑岩,开展锆石U-Pb年龄和Hf同位素的研究工作,获得了流纹斑岩锆石206Pb/238U的加权平均年龄为128±1Ma (n=14,MSWD=2.5),锆石Hf同位素(176Hf/177Hf),和εHf(t)分别为0.28247～0.28262和-2.5～ -7.7;花岗斑岩锆石206 Pb/238U的加权平均年龄为129±1Ma (n=12,MSWD=1.3),锆石Hf同位素(176Hf/177Hf),和εHf(t)分别为0.28239～0.28259和-3.6～-10.7.本文确定金牛盆地中流纹斑岩和花岗斑岩的形成时代为129～128Ma,与金牛盆地火山岩的形成时代(130～125Ma)相当,暗示金牛盆地火山岩和次火山岩均形成于早白垩世,流纹斑岩和花岗斑岩是壳幔混合的产物.金牛盆地火山岩的形成时代与宁芜、、庐枞盆地相当,次火山岩的形成时代也与宁芜、庐枞盆地玢岩铁矿的成矿岩体相当.提出金牛盆地除应关注玢岩铁矿外,更应该关注与次火山岩有关的热液金矿.     

庐枞盆地与A型花岗岩有关的磁铁矿-阳起石-磷灰石矿床——以马口铁矿床为例

庐枞中生代火山岩盆地位于长江中下游断陷带内,地处扬子板块的北缘.庐枞盆地内的火山岩和侵入岩分布广泛,包括龙门院、砖桥、双庙和浮山四组火山岩以及34个侵入岩体.最近在庐枞盆地南部正长岩中发现一种新类型铁矿床——马口铁矿床.本次工作通过详细野外地质和室内研究,系统开展了马口铁矿床矿床地质特征、成矿年代学和成矿流体特征研究.马口铁矿床的矿体沿石英正长斑岩体中构造破碎带产出,产状严格受构造破碎带控制,矿床围岩蚀变晕范围较为局限.矿床的成矿作用可分为碱性长石阶段、磁铁矿阶段、石英-硫化物阶段和碳酸盐阶段,其中磁铁矿阶段是主要成矿阶段,形成与宁芜玢岩型铁矿床相类似磁铁矿-磷灰石-阳起石三矿物组合.矿床中与磁铁矿共生的磷灰石流体包裹体均一温度范围为252.2 ～ 322.6℃,反映其成矿温度略低于典型的玢岩铁矿床.通过赋矿岩体锆石LA-ICP-MS U-Pb定年和金云母Ar-Ar方法,确定马口铁矿床的成矿时代为127.3 ±0.8Ma,赋矿岩体的形成时代为129.4±1.4Ma,结合矿床地质特征,可以得出矿区内石英正长斑岩只是提供了赋矿空间,矿床的形成与其无直接成因联系,而可能与黄梅尖A型花岗岩体有关.马口铁矿床是庐枞盆地成矿作用最后阶段的产物,也可能是长江中下游成矿带中最晚一次成矿事件的代表,具有显著的特色.     

华南富铀花岗岩和产铀花岗岩特征

文章简要叙述了我国南方5省(区)富铀花岗岩和产铀花岗岩概况,详细讨论了产铀花岗岩的主要特征:断裂构造发育,蚀变作用非常强烈,经常有二云母花岗岩出现,中基性和酸性脉岩比较发育。正是这些特征,决定了它们产出铀矿床的能力。     

Collision‐related granite magma genesis,potential sources and tectono‐magmatic evolution: comparison between central,northwestern and western Anatolia (Turkey)

The central, northwestern and western Anatolian magmatic provinces are defined by a large number of late Mesozoic to late Cenozoic collision‐related granitoids. Calc‐alkaline, subalkaline and alkaline intrusive rocks in central Anatolia are mainly metaluminous, shoshonitic, I‐ to A‐types. They cover a petrological range from monzodiorite through quartz monzonite to granite/syenite, and are all enriched in LILE. Their geochemical characteristics are consistent with formation from a subduction‐modified mantle source. Calc‐alkaline plutonic rocks in northwestern Anatolia are mainly metaluminous, medium‐ to high‐K and I‐types. They are monzonite to granite, and all are enriched in LILE and depleted in HFSE, showing features of arc‐related intrusive rocks. Geochemical data reveal that these plutons were derived from partial melting of mafic lower crustal sources. Calc‐alkaline intrusive rocks in western Anatolia are metaluminous, high‐K and I‐types. They have a compositional range from granodiorite to granite, and are enriched in LILE and depleted in HFSE. Geochemical characteristics of these intrusive rocks indicate that they could have originated by the partial melting of mafic lower crustal source rocks.     

Age,origin, and tectonic implications of Palaeozoic rapakivi granites in the North Qaidam orogen,Northwest China

Palaeozoic rapakivi granites occur in the western segment of the China Central Orogenic System. Exhibiting typical rapakivi texture, these granites contain magmatic microgranular enclaves of intermediate compositions. SHRIMP zircon U–Pb ages for the granites and enclaves are 433 ± 5 Ma and 433 ± 3 Ma, respectively. The rapakivi granites are magnesian to ferroan, calc-alkalic to alkalic, and are characterized by high FeO_t/(FeO_t + MgO) (0.74–0.91) and Ga/Al ratios, and SiO₂, Na₂O + K₂O and rare earth element (apart from Eu) contents, but low CaO, Ba, and Sr contents. These are typical A-type granite geochemical features. The granites and enclaves exhibit a uniform decrease in TiO₂, CaO, Na₂O, K₂O, FeO, and MgO with increasing SiO₂, and both lithologies have similar trace element patterns. Whole-rock ?_Nd(t) values vary from??9.2 to??8.7 for the granites and from??9.0 to??8.4 for the enclaves, but zircon ?_Hf(t) values vary more widely from??5.8 to??0.2 and??4.6 to +5.1, respectively. Our data suggest that the granites and enclaves have crystallized from different magmas. The granites appear to have been derived from old continental crust, whereas the enclaves required a source having a juvenile component. The spherical shape and undeformed nature of the granites and their geochemical characteristics, coupled with the (ultra)-high pressure metamorphism and evolution of Palaeozoic granitoid magmatism in the North Qaidam orogen, indicate that the rapakivi granites were generated in a post-collisional setting. These rocks are therefore an example of Palaeozoic rapakivi granites emplaced in a post-collisional, extensional orogenic setting.     

Laser-ICP-MS U–Pb zircon ages and geochemical and Sr–Nd–Pb isotopic compositions of the Niyasar plutonic complex,Iran: constraints on petrogenesis and tectonic evolution

We conducted geochemical and isotopic studies on the Oligocene–Miocene Niyasar plutonic suite in the central Urumieh–Dokhtar magmatic belt, in order better to understand the magma sources and tectonic implications. The Niyasar plutonic suite comprises early Eocene microdiorite, early Oligocene dioritic sills, and middle Miocene tonalite + quartzdiorite and minor diorite assemblages. All samples show a medium-K calc-alkaline, metaluminous affinity and have similar geochemical features, including strong enrichment of large-ion lithophile elements (LILEs, e.g. Rb, Ba, Sr), enrichment of light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs, e.g. Nb, Ta, Ti, P). The chondrite-normalized rare earth element (REE) patterns of microdiorite and dioritic sills are slightly fractionated [(La/Yb)_n = 1.1–4] and display weak Eu anomalies (Eu/Eu* = 0.72–1.1). Isotopic data for these mafic mantle-derived rocks display I_Sr = 0.70604–0.70813, ?_Nd (microdiorite: 50 Ma and dioritic sills: 35 Ma, respectively) = +1.6 and ?0.4, TDM = 1.3 Ga, and lead isotopic ratios are (²⁰⁶Pb/²⁰⁴Pb) = 18.62–18.57, (²⁰⁷Pb/²⁰⁴Pb) = 15.61–15.66, and (²⁰⁸Pb/²⁰⁴Pb) = 38.65–38.69. The middle Miocene granitoids (18 Ma) are also characterized by relatively high REE and minor Eu anomalies (Eu/Eu* = 0.77–0.98) and have uniform initial ⁸⁷Sr/⁸⁶Sr (0.7065–0.7082), a range of initial Nd isotopic ratios [?Nd(T)] varying from ?2.3 to ?3.7, and Pb isotopic composition (²⁰⁶Pb/²⁰⁴Pb) = 18.67–18.94, (²⁰⁷Pb/²⁰⁴Pb) = 15.63–15.71, and (²⁰⁸Pb/²⁰⁴Pb) = 38.73–39.01. Geochemical and isotopic evidence for these Eocene–Ologocene mafic rocks suggests that the magmas originated from lithospheric mantle with a large involvement of EMII component during subduction of the Neotethyan ocean slab beneath the Central Iranian plate, and were significantly affected by crustal contamination. Geochemical and isotopic data of the middle Miocene granitoids rule out a purely crustal-derived magma genesis, and suggest a mixed mantle–crustal [MASH (melting, assimilation, storage, and homogenization)] origin in a post-collision extensional setting. Sr–Nd isotope modelling shows that the generation of these magmas involved ～60% to 70% of a lower crustal-derived melt and ～30% to 40% of subcontinental lithospheric mantle. All Niyasar plutons exhibit transitional geochemical features, indicating that involvement of an EMII component in the subcontinental mantle and also continental crust beneath the Urumieh–Dokhtar magmatic belt increased from early Eocene to middle Miocene time.