桐柏造山带古元古代变质基性岩的锆石U-Pb年龄及其地质意义

桐柏造山带位于秦岭和大别造山带之间,其北侧对应于北秦岭中高级变质杂岩,南侧对应于大别山高压/超高压变质杂岩.锆石U-Pb定年在一个榴辉岩中获得其原岩侵位年龄和锆石重结晶或生长年龄分别为(1961±23) Ma和(1949±51) Ma,在一个石榴角闪岩(退变榴辉岩？)中获得其变质年龄为(1936±26) Ma,表明这两种基性岩石在二叠-三叠纪高压变质之前经历了古元古代变质作用的影响.由桐柏—大别—苏鲁造山带可利用的年代学资料确定,扬子陆块北缘发育的古元古代构造热事件可分为两幕,第一幕发生在约1.97～1.93 Ga,可能与Columbia超大陆的聚合有关;第二幕发生在约1.85～1.82 Ga,或者代表Columbia超大陆汇聚的继续,或者是陆块边缘增生过程的结果.     

江南古陆中段沧水铺群锆石U-Pb年龄和构造演化意义

江南古陆中段湖南益阳地区沧水铺群发育在冷家溪群和板溪群之间,其在地层柱中的位置一直是中国地质学家解疑江南造山带何时启动的关键层位。笔者在沧水铺群中的火山集块岩中获得锆石SHRIMP U-Pb年龄(821±13)Ma,再次验证了沧水铺群中的火山岩是820 Ma武陵运动之前的火山事件的产物,与武陵运动的启动有关,也说明该期火山岩与冷家溪群中大量的火山凝灰岩为同一构造运动的产物。     

南天山南缘下二叠统玄武质岩浆的地球化学特征及其构造意义

南天山造山带南缘发育大量的下二叠统火山岩。地球化学指标显示南天山南缘西段拜城县宿相厄肯沟出露的下二叠统小提坎立克组玄武岩为拉斑玄武岩(里特曼指数δ=2.93)。 该套岩石低FeO^T、MgO和全碱(Na₂O+K₂O),高CaO和Al₂O₃,相对高钠,低钾; Rb、K、U和Ba 等大离子亲石元素富集; 稀土配分曲线整体协调一致,为向右缓倾的曲线,轻稀土元素(LREE)相对富集; 微量元素配分型式呈"驼峰状"; 玄武岩的母岩浆经历了以单斜辉石为主的分离结晶作用和强烈的陆壳混染作用。同位素组成 ¹⁴³ Nd/¹⁴⁴ Nd =0.512 849±0.000 005, ⁸⁷ Sr/⁸⁶ Sr =0.706 845±0.000 014; -10<ε_Nd(t)<0,且ε_Sr(t)>0,模式年龄T_DM 为1.463 279 765 Ga。 结合区域地质资料表明,小提坎立克组玄武岩形成于板内伸展环境,具有主动裂谷作用性质,说明南天山造山带南缘当时处于碰撞后伸展背景。     

西秦岭楔的构造属性及其增生造山过程

西秦岭楔是叠置于早古生代造山作用基础上形成的并插入祁连和昆仑早古生代造山带内部的楔形地质体,以大面积出露三叠系并发育多条蛇绿混杂岩带、大型韧性剪切带、中生代火山-岩浆作用和斑岩-矽卡岩型矿床为典型特征,具有增生造山作用的典型特征。这些蛇绿混杂岩带和岛弧钙碱性火山-岩浆岩的形成时代均具有向南逐渐变年轻的空间演化特征,显示了特提斯洋演化过程中海沟具有向南撤退的基本特征。砂岩碎屑组成以及源区特征研究结果表明,西秦岭楔三叠系形成于活动大陆边缘,其碎屑沉积物来自于古特提斯洋北侧的增生杂岩及岛弧。丰富的岛弧钙碱性火山-岩浆岩和沉积组合以及赋存的斑岩-矽卡岩型矿床,均与东昆仑及南秦岭相一致,呈现出相似的岩石组合类型以及岩石地球化学和同位素地球化学特征。这些事实表明,三叠纪时期,东昆仑、西秦岭以及祁连造山带是一个有机整体,自西向东存在一条三叠纪增生岩浆弧。锆石Hf同位素及岩石地球化学成分结果则表明,该增生岩浆弧部分岩浆来自于俯冲增生杂岩的部分熔融。     

江南造山带西南段梵净山地区镁铁质-超镁铁质岩:形成时代、地球化学特征与构造环境

梵净山地区位于江南造山带的西南缘,这里新元古代的镁铁质-超镁铁质岩浆岩广泛发育,岩性包括枕状熔岩、超镁铁质-镁铁质岩床群以及浅成侵入的辉长岩,成分属拉斑玄武岩系列。其中枕状熔岩以富集轻稀土元素和Rb、Ba、Th、U等强不相容元素,亏损高场强元素Nb和Ta,低的εNd(t)值为特征,明显不同于洋脊玄武岩,推测其成因可能与富集型地幔的部分熔融有关,形成于与俯冲有关的弧后小洋盆环境。超镁铁质-镁铁质岩床群主要由辉绿岩和碳酸辉橄岩组成,其中超镁铁质岩床群中出现大量的原生碳酸盐矿物,指示它们形成于拉张(甚至裂谷)的构造环境。辉长岩可能是区内最晚形成的岩浆岩,其SHRIMP锆石U-Pb年龄为821±4Ma。由枕状熔岩经超镁铁质-镁铁质岩床群到辉长岩,高场强元素Nb和Ta的亏损程度减弱、轻稀土元素的富集程度降低、εNd(t)值由负值变为正值,指示随时间的由早到晚,来自亏损地幔的物质不断增加。推测梵净山地区新元古代岩浆作用的顺序大致为:枕状熔岩(～840Ma)→白云母花岗岩(～838Ma)→碳酸超镁铁质岩床群→镁铁质岩床群→辉长岩(～821Ma),构造环境由俯冲-碰撞到拉张-裂谷。     

青藏高原南部与东南部重要成矿带的大地构造定格与找矿前景

印度/亚洲汇聚-碰撞过程经历了新特提斯洋盆滋生、消减和俯冲、亚洲南缘增生造山以及印度/亚洲碰撞造山和青藏高原的隆升,在青藏高原南部和东南部造就了"冈底斯火山岩浆带"、"雅鲁藏布江缝合带"、"喜马拉雅碰撞造山带"和大量物质向南东逃逸的"三江侧向挤出地体群",以及相应形成具有重大找矿突破战略前景的"冈底斯成矿带"、"雅鲁藏布江成矿带"、"特提斯喜马拉雅成矿带"和"三江成矿带"。本文通过对四大成矿带的大地构造定格讨论了与资源前景相关的科学问题,提出"冈底斯成矿带"中的岛弧型斑岩铜金矿具有找矿的重大潜力、重视藏东—滇西地区的俯冲-碰撞型岩浆成矿专属性研究;提出扩大西藏罗布莎铬铁矿矿集区的开发规模,以及在西部阿里地区的大型超基性岩体中寻找新的铬铁矿远景地的思路;在三江多阶段成矿作用的叠合型矿床中,集中古特提斯和新特提斯成矿类型,关注与斜向碰撞有关的走滑剪切带对成矿作用的制约机制;需进一步确定特提斯喜马拉雅矿化带与藏南拆离系关系和重视始—中新世高Sr/Y花岗(斑)岩的成矿专属性及找矿前景。     

Petrogenesis and tectonic implications of early Silurian high-K calc-alkaline granites and their potassic microgranular enclaves,western Kunlun orogen,NW Tibetan Plateau

The western Kunlun orogen occupies a key position along the tectonic junction between the Pan-Asian and Tethyan domains, reflecting Proto- and Palaeo-Tethys subduction and terrane collision during early Palaeozoic to early Mesozoic time. We present the first detailed zircon U–Pb chronology, major and trace element, and Sr–Nd–O–Hf isotope geochemistry of the Qiukesu pluton and its microgranular enclaves from this multiple orogenic belt. SHRIMP zircon U–Pb dating shows that the Qiukesu pluton was emplaced in the early Silurian (ca. 435 Ma). It consists of weakly peraluminous high-K calc-alkaline monzogranite and syenogranite, with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7131–0.7229, ?_Nd(T) of –4.1 to –5.7, δ¹⁸O of 8.0–10.8‰, and ?_Hf(T) (in situ zircon) of –4.9. Elemental and isotopic data suggest that the granites formed by partial melting of lower-crustal granulitized metasedimentary-igneous Precambrian basement triggered by underplating of coeval mantle-derived enclave-forming intermediate magmas. Fractional crystallization of these purely crustal melts may explain the more felsic end-member granitic rocks, whereas such crustal melts plus additional input from coeval enclave-forming intermediate magma could account for the less felsic granites. The enclaves are intermediate (SiO₂ 57.6–62.2 wt.%) with high K₂O (1.8–3.6 wt.%). They have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7132–0.7226, ?_Nd(T) of –5.0 to –6.0, δ¹⁸O of 6.9–9.9‰, and ?_Hf(T) (in situ zircon) of –8.1. We interpret the enclave magmas as having been derived by partial melting of subduction-modified mantle in the P–T transition zone between the spinel and spinel-garnet stability fields. Our new data suggest that subduction of the Proto-Tethyan oceanic crust was continuous to the early Silurian (ca. 435 Ma); the final closure of the Proto-Tethys occurred in the middle Silurian.     

Gold Deposits in the Sanandaj–Sirjan Zone: Orogenic Gold Deposits or Intrusion‐Related Gold Systems?

Most of the known large gold deposits in Iran are located along the Sanandaj–Sirjan Zone, western Iran, which hosts a wide range of gold deposit types. Gold deposits in the belt, hosted in upper Paleozoic to upper Mesozoic volcano‐sedimentary sequences of lower greenschist to lower amphibolite metamorphic grade, appear to represent mainly orogenic and intrusion‐related gold deposit types. The largest resource occurs at Muteh, with smaller deposits/occurrences at Zartorosht, Qolqoleh, Kervian, Qabaqloujeh, Kharapeh, and Astaneh. Although a major part of the gold deposits in the Sanandaj–Sirjan Zone are related to metamorphic devolatilization, some deposits including Muteh and Astaneh are related to short‐lived disruptions in an extensional tectonic regime and are associated with magma generation and emplacement. The age of gold ore formation in the orogenic gold deposits is Late Cretaceous to Tertiary, reflecting peak‐metamorphism during regional Cretaceous–Paleocene convergence and compression. The Oligocene to Pliocene age of most intrusion‐related gold systems is consistent with the young structural setting of the gold ore bodies; these deposits are sequestered along normal faults, correlated with Middle to Late Tertiary extensional tectonic events. This relationship is comparable to the magmatic‐metallogenetic evolution of the Urumieh‐Dokhtar magmatic arc, where the number of different types of gold‐copper deposits and the magnitude of the larger ones followed development of a magmatic arc. The appropriate explanation may be related to two different stages of gold mineralization consisting of a first compressional phase during the Late Cretaceous to Early‐Middle Tertiary, which is related to orogenic gold mineralization in the Qolqoleh, Kervian, Qabaqloujeh, Kharapeh, and Zartorosht deposits, and the extensional phase during the Eocene to Pliocene that is recognized by young intrusion‐related gold mineralization in the Muteh and Astaneh deposits.     

Early Permian extensional shearing of an Ordovician granite: The Saint-Eutrope “C/S-like” orthogneiss (Montagne Noire,French Massif Central)

Dating the magmatic events in the Montagne Noire gneiss dome is a key point to arbitrate between the different interpretations of the Late Carboniferous–Early Permian tectonics in this southern part of the Variscan belt. The Saint-Eutrope orthogneiss crops out along the northern flank of the dome. We show that the protolith of this orthogneiss is an Ordovician granite dated at 455 ± 2 Ma (LA-ICP-MS U-Pb dating on zircon). This age is identical to that previously obtained on the augen orthogneiss of the southern flank, strongly suggesting that both orthogneiss occurrences have the same Ordovician protolith. The Saint-Eutrope orthogneiss experienced intense shearing along the Espinouse extensional detachment at ca. 295 Ma (LA-ICP-MS U-Pb-Th on monazite), an age close to that determined previously on mica by the ³⁹Ar-⁴⁰Ar method and contemporaneous with the emplacement age of the syntectonic Montalet granite farther to the west. This normal sense shearing reworked previous fabrics related to Variscan thrusting that can be still observed in the augen orthogneiss of the southern flank, and is responsible for the spectacular “C/S-like” pattern of the Saint-Eutrope orthogneiss. This work also shows that care is needed when dealing with C/S-type structures, since they can develop not only in syntectonic intrusions, but also in orthogneisses affected by an intense secondary deformation, at decreasing temperature.