中国东部新生代玄武岩中单斜辉石巨晶的主要特征及成因信息

作者通过对我国东部新生代玄武岩中巨晶Cpx的109个样品的化学成分的统计、计算及投影,研究了Cpx巨晶的产状、物性及矿物化学的主要特征,并对其成因信息提出了自己的看法。Cpx巨晶的成分以A1_2O_3、A1~(v1)较高而Ti、Mg'中等为特征。但在“反应边”中,A1_2O_3、Na_2O、A1、A1~(v1)则明显降低。我国巨晶以普通辉石为主,而透辉石、次透辉石及顽透辉石少见。国内不同地区的Cpx巨晶成分也略有不同:华南区巨晶的Wo较高而A1_2O_3较低;华北区Fs、A1_2O_3较高;而东北区En较高而A1_2O_3中等。有关Cpx巨晶的成因信息,作者的观点是:(1)巨晶Cpx是寄主玄武岩浆在上地幔中结晶的产物,而不是地幔解体的捕虏晶。(2)我国巨晶Cpx的结晶温度是1021—1209℃,压力为1.4—3.2GPa,所以,Cpx巨晶的结晶深度为48—104Km。(3)我国东部新生代玄武岩均产于大陆板块内部,以碱性玄武岩成分为主,而拉斑玄武岩较少。     

阿尔泰东部新生代火山岩的地球化学特点及构造环境

新疆三大山系均有新生代火山岩,其中南部西昆仑、阿尔金山及天山托云等地的新生代火山岩比较发育,在新疆东北部地区的阿尔泰哈拉乔拉一带也发育新生代的陆内喷发火山岩。这套火山岩虽然面积不是很大,但具有大陆溢流玄武岩的特点。在地球化学上也具有典型热点成因洋岛玄武岩或其它地幔柱成因玄武岩的特点,如TiO2含量高大于2%、轻稀土元素和不相容元素显著富集等。表明新疆北部特别是阿尔泰造山带东部演化到新生代以后,有向裂陷拉张方向演化的趋势,标志着一个新的大地构造演化阶段的到来。     

威宁铜厂河玄武岩铜矿成矿地质特征及成因探讨

研究威宁铜厂河铜矿区玄武岩的岩石类型、常量元素、微量元素及稀土元素地球化学特征,探讨玄武岩铜矿的成矿地质背景、控矿因素、产出特征和成矿规律,系统总结了找矿标志.研究认为,本区玄武岩具大陆拉斑玄武岩特征,铜矿属火山同生热液成矿叠加埋藏变质作用改造形成的玄武岩铜矿.     

Triassic mid-oceanic sedimentation in Panthalassa Ocean: Sambosan accretionary complex, Japan

Abstract   The Sambosan accretionary complex of southwest Japan was formed during the uppermost Jurassic to lowermost Cretaceous and consists of basaltic rocks, carbonates and siliceous rocks. The Sambosan oceanic rocks were grouped into four stratigraphic successions: (i) Middle Upper Triassic basaltic rock; (ii) Upper Triassic shallow-water limestone; (iii) limestone breccia; and (iv) Middle Middle Triassic to lower Upper Jurassic siliceous rock successions. The basaltic rocks have a geochemical affinity with oceanic island basalt of a normal hotspot origin. The shallow-water limestone, limestone breccia, and siliceous rock successions are interpreted to be sediments on the seamount-top, upper seamount-flank and surrounding ocean floor, respectively. Deposition of the radiolarian chert of the siliceous rock succession took place on the ocean floor in Late Anisian and continued until Middle Jurassic. Oceanic island basalt was erupted to form a seamount by an intraplate volcanism in Late Carnian. Late Triassic shallow-water carbonate sedimentation occurred at the top of this seamount. Accumulation of the radiolarian chert was temporally replaced by Late Carnian to Early Norian deep-water pelagic carbonate sedimentation. Biotic association and lithologic properties of the pelagic carbonates suggest that an enormous production and accumulation of calcareous planktonic biotas occurred in an open-ocean realm of the Panthalassa Ocean in Late Carnian through Early Norian. Upper Norian ribbon chert of the siliceous rock succession contains thin beds of limestone breccia displaced from the shallow-water buildup resting upon the seamount. The shallow-water limestone and siliceous rock successions are nearly coeval with one another and are laterally linked by displaced carbonates in the siliceous rock succession.     

准噶尔盆地陆梁地区基底火山岩的岩石地球化学及其构造环境

准噶尔盆地腹地陆梁起基底火山岩岩性为富钠玄武岩及流纹岩，总体显示出板内双峰火山岩特点。玄武岩的特征是：岩石的五晶和基质中普遍出现橄榄石；辉石为普通辉石；斑晶和基质中的长石为偏酸性的斜长石（平均牌号为30－50）；全岩化学成分CIPW计算结果表明，绝大部分含有Ne（2．8％－4．6％），均含有O1（19．3％－10．1％）和Di（0．2％－24．6％），标准矿物分子组合为Ne＋O1＋Di＋An；在全碱－SiO2图上玄武岩投影于碱性区；Mg^#＜65；REE总量为110．29－158．06μg/g。（La/Y）N变化范围为3．10－4．51。δEu变化于0．93－1．04；弱武岩的微量元素标准化图解为LILE相对于LREE适度富集，Nb,Ta相对于LREE和LILE亏损。Ni,Cr含量略低于原始岩浆的参考值；以上特征表明，弱武岩总体上属于碱性橄榄玄武岩；玄武岩具有较同正的εNd(t)和低的^87Sr/^86Sr，而流纹岩则具有较低的εNd(t)和较高的^87Sr/^86Sr，反映它们的同源性和遭受陆壳物质同化混染程度的不同。同位素Rb-Sr等时线年龄和单颗粒锆石蒸发年龄集中在323－395Ma。以上特点表明，陆梁玄武岩来自于亏损的地幔源区，并经历了一定程度的分异作用和陆壳物质的混染作用，其形成于板内环境，与泥盆纪－石灰纪区域伸展作用有关，因此，陆梁隆起带基底很可能是一个大陆裂谷带。     

Generation of Deccan Trap magmas

Deccan Trap magmas may have erupted through multiple centers, the most prominent of which may have been a shield volcano-like structure in the Western Ghats area. The lavas are predominantly tholeiitic; alkalic mafic lavas and carbonatites are rare. Radioisotope dating, magnetic chronology, and age constraints from paleontology indicate that although the eruption started some 68 Ma, the bulk of lavas erupted at around 65–66 Ma. Paleomagnetic constraints indicate an uncertainty of ± 500,000 years for peak volcanic activity at 65 m.y. in the type section of the Western Ghats. Maximum magma residence times were calculated in this study based on growth rates of “giant plagioclase” crystals in lavas that marked the end phase of volcanic activity of different magma chambers. These calculations suggest that the > 1.7 km thick Western Ghats section might have erupted within a much shorter time interval of ∼ 55,000 years, implying phenomenal eruption rates that are orders of magnitude larger than any present-day eruption rate from any tectonic environment. Other significant observations/conclusions are as follows: (1) Deccan lavas can be grouped into stratigraphic subdivisions based on their geochemistry; (2) While some formations are relatively uncontaminated others are strongly contaminated by the continental crust; (3) Deccan magmas were produced by 15–30% melting of a Fe-rich lherzolitic source at ∼ 3–2 GPa; (4) Parent magmas of the relatively uncontaminated Ambenali formation had a primitive composition with 16%MgO, 47%SiO₂; (5) Deccan magmas were generated much deeper and by significantly more melting than other continental flood basalt provinces; (6) The erupted Deccan tholeiitic lavas underwent fractionation and magma mixing at ∼ 0.2 GPa. The composition and origin of the crust and crust/mantle boundary beneath the Deccan are discussed with respect to the influence of Deccan magmatic episode.     

新生代黄骅坳陷构造伸展、沉积作用和岩浆活动

黄骅坳陷是一典型的张性盆地,其构造主要受西缘向东倾的拆离断层的控制。构造沉降分析显示盆地西部以裂谷沉降为主,而向东热沉降逐渐增大,反映了上地壳的强烈裂谷区与岩石圈深部强烈减薄区发生了错位。沉积作用在裂谷期和热沉降期具明显不同的特点,指示了早期基底快速不均匀的下沉和晚期基底广泛缓慢下沉的过程。玄武岩的形成和演化也与盆地构造发展过程相一致。研究结果表明,黄骅坳陷是在下伏岩石圈上部发生简单剪切和下部发生纯剪切的构造伸展过程中形成的。     

Permian radiolarians,chert and basalt from the Daxinshan Formation in Lancangjiang belt of southwestern Yunnan,China

The stratigraphical sequences composed of chert and basalt were found in the Daxinshan area of Simao and the Manbie area of Jinghong, southwestern Yunnan. The Middle Permian to ealiest Late Permian radiolarians, such as Follicucullus and Pseudoalbaillella, have been identified from the chert. The chert from the Manbie area of Jinghong is characterized by high SiO₂content (over 92%), large ratios of MnO/TiO₂ (2.15) and low ratios of Al/(Al+Fe+Mn) (≤0.1) and Ce/Ce*(0.4), which indicate that the chert was deposited in pelagic basin. The chert from the Daxinshan area of Simao, however, is characterized by low SiO₂ content, low ratios of MnO/TiO₂ (0.27) and high ratios of Al/(Al+Fe+Mn) (0.49) and Ce/Ce*(0.88), which imply that the chert was deposited in continental margin basin. The basalts from the both areas belong to tholeiite series, and the chemical compositions of their major, rare earth and trace elements show the characteristics of MORB. These results evidence that there are volcanic rocks and chert sequences representing pelagic basin and oceanic basin near continent. These sequences and the formerly reported island-arc volcanic rock sequences imply that the Daxinshan Formation in the Lancangjiang belt represents a sedimentary assemblage formed in active continental margin basin.     

新疆北部的两类埃达克岩

新疆北部有两类埃达克岩，一是俯冲型，形成于早、中泥盆世-早石炭世晚期（≥320Ma），包括了埃达克岩、富Nb玄武岩、高（富）Mg安山岩。第二类埃达克岩是底侵型，形成于中晚二叠世（≤280Ma）。第一类埃达克岩分布于西天山的阿拉套山、博罗科努山，中天山的骆驼沟和巴仑台，东天山的土屋-延东，阿尔泰山陆缘南富蕴-青河南，准噶尔盆地中部陆梁，克拉玛依等地。在阿尔泰陆缘南，苦橄岩与埃达克岩、富Nb玄武岩和高（富）Mg安山岩密切组合。第二类埃达克岩分布于西天山的阿吾拉勒山和东天山的三岔口，未发现富Nb玄武岩和高（富）Mg安山岩组合。俯冲型埃达克岩、富Nb玄武岩和高（富）Mg安山岩的高Sr低Y、Yb、富Eu及高εNd（t）（＋1．5～＋10．0），低（^87Sr／^86Sr）；（〈0．7070）的同位素组成，均一致表明其源区物质为洋壳板片，部分为地幔楔、弧前棱柱，产于岛弧环境；而底侵型埃达克岩源于底侵的幔源玄武质物质，形成于后造山环境。两类埃达克岩及其组合岩石的地质及地球化学特点，展示了中亚型造山在本区晚古生代陆壳增生作用的多样性：在增生构造过程上，有洋壳板片的斜俯冲、俯冲板片的撕裂、板片窗、俯冲剥蚀及玄武质物质的底侵作用等；在增生方向上，有洋壳板片的侧向斜俯冲，也有玄武质物质垂向上底侵于壳-幔边界；在增生物质上，有洋壳板片、地幔楔、受地幔楔混染的洋壳板片熔体，弧前棱柱、地幔楔受板片熔体交代后形成熔体及底侵的幔源玄武质物质。与两类埃达克岩有关，尤其是第一类埃达克岩及其组合岩石，在本区广泛发育了Cu、Au成矿作用，其中部分达到大型-超大型规模。因此．对埃达克岩及其组合岩石的识别及相关Cu、Au成矿作用的找矿勘探应予以足够重视。     

内蒙古温都尔庙和巴彦敖包-交其尔蛇绿岩的元素与同位素地球化学：对古亚洲洋东部地幔域特征的限制

内蒙古中部发育的三条蛇绿岩带是华北板块和西伯利亚板块之间的缝合带。本文系统研究了其中的温都尔庙和巴彦敖包-交其尔两个蛇绿岩带中变质玄武岩的元素和 Sr、Nd、Pb 同位素地球化学。苏右旗温都尔庙碱性玄武岩为轻稀土富集型;岩石具有板内和大陆裂谷区玄武岩的特征,可能代表了600Ma 左右,温都尔庙地区开始发育的新洋盆。采自苏左旗的巴彦敖包-交其尔玄武岩分为两类,一类呈现轻稀土富集型,呈洋岛玄武岩特征;另一类具有明显的 Nb、Ta 负异常,显示大洋岛弧玄武岩特征,洋岛玄武岩的存在表明古亚洲洋曾经发育洋盆,大洋岛弧玄武岩的存在表明古亚洲洋内部有大洋岩石圈之间的俯冲。将本文的古亚洲洋洋岛玄武岩与中国西南地区的特提斯洋岛玄武岩进行系统的元素和同位素地球化学特征对比表明,古亚洲洋的洋岛玄武岩显示高 U/Pb(HU)和北大西洋和太平洋省的特征,而特提斯洋岛玄武岩属于印度洋省。这些说明古亚洲洋地幔域与特提斯地幔域是两个独立的构造域,它们代表了长期演化的两个不同的地幔地球化学域。