Geochemical discrimination of metabasalt rocks of the Fan–Karategin transitional blueschist/greenschist belt, South Tianshan, Tajikistan: seamount volcanism and accretionary tectonics

The Fan–Karategin metamorphic belt, South Tianshan, Tajikistan, is regarded to be an ancient subduction–accretionary complex and is composed of three tectonostratigraphic units which display lithologies consistent with different tectonic settings. The mafic schists, which make up the major part of the older unit of the belt, contain both alkali and tholeiitic metabasalts. On the basis of rare-earth and other immobile element characteristics, the alkali metabasalts are akin to within-plate ocean island basalts, whereas the tholeiitic metabasalts resemble E-type MORB. The association is interpreted to have been formed on seamount-like structures under a within-plate plume. Bedded cherts and marbles in the unit are regarded as ancient pelagic sediments and carbonate caps developed upon basaltic seamounts, respectively. Dismemberment of the seamount-related basalts and pelagic sediments and the high-P/low-T prograde metamorphism of the unit rocks up to transitional blueschist/greenschist facies was the result of paleoseamount submergence into a subduction zone. This unit is tectonically overlain by arc-derived metavolcanic unit and a disrupted, mainly clastic unit of Upper Ordovician–Lower Silurian age. Metavolcanic and metasedimentary rocks of the two upper units have geochemical characteristics compatible with subduction-related origin. The lithological assemblages of the individual units and their juxtaposition suggest an origin involving collision–accretionary processes. The Fan–Karategin belt is a subduction–accretionary complex which formed during subduction of oceanic crust under a volcanic arc and was subjected to tectonic juxtaposition and imbrication of seamount, deep-sea, trench and volcanic arc sequences.     

Rapid facies changes in Holocene fissure ridge hot spring travertines, Rapolano Terme, Italy

Holocene hot water travertine continues to form at Terme San Giovanni, near Rapolano Terme, central Italy, although artificial diversion of the water has reduced deposition. Mesothermal water (≈38–39 °C) emerging from fault-controlled vents located on a hilltop has created a linear fissure ridge 240 m long and up to 10 m high. Active parts of the ridge crest are covered by small cones; inactive parts are locally neotectonically fissured and have small pools. Ridge deposits include crystalline crust, paper-thin raft and shrub lithotypes. The ridge has both smooth and terraced marginal slopes, dominated by crystalline crusts with small shrubs in terrace pools. At the base of the ridge, there is a rapid transition to lateral flats and depressions, where water from the ridge collects and deposits shrub, irregular pisoid, reed, paper-thin raft and fine-grained and organic-rich travertines. Water channelled to nearby valley sides deposits thick crystalline crusts on valley slopes and waterfall overhangs, locally with small pools filled by smooth spherical pisoids. On the valley floor, mixing of waters forms varied stream-fill deposits that include micritic reed, paper-thin raft and coated bubble travertines. The diversity of travertine facies observed results from the location of the Terme San Giovanni hot springs on a hill crest, thus providing a wide array of downslope locations for further deposition. The abrupt facies transitions observed are characteristic of hot spring carbonates and result from a combination of rapid decrease in precipitation away from vents, variations in local surface topography and the feedback effect of travertine deposition itself, which dams and diverts water flow.     

山东蓬莱新生代玄武岩的构造背景及宝石找矿前景

山东蓬莱位于胶东地体北缘中部。板块动力作用形成了前古生代近EW向构造和中生代NE—NNE向走滑断层,侏罗纪以来地幔柱构造作用逐渐加强,地壳伸展减薄,幔源物质上涌,导致新生代碱性玄武岩喷发。碱性玄武岩中普通辉石、镁铝榴石等可作为低档宝石原料。玄武岩的构造背景、喷发形式、火山机构、岩性特征等与昌乐赋蓝宝石的碧玄岩极为相似,经过有针对性的详细调查研究,寻找红、蓝宝石是有希望的。     

中国新生代碱性玄武岩型宝石矿床

中国东部分布着大量的新生代玄武岩,其中第三纪的中新世和上新世的碱性玄武岩是蓝宝石、红宝石、红色锆石、橄榄石、镁铝榴石、月光石和辉石类宝石的寄主岩石,已在其内发现了多种宝石的原生矿床和矿化点。今后应重视碱性玄武岩型宝石矿床的研究和找矿评价工作。     

塔里木盆地北缘新生代碱性玄武岩的地质地球化学

新生代碱性玄武岩是塔里木盆地北缘碱性岩带最晚期形成的,由第三系盆地中陆相中心式喷发的碱性橄榄玄武岩和其下古生代地层中的碱性橄榄辉长岩岩筒和岩脉组成。它们是来自富集地幔的同源碱性岩浆,在不同空间冷凝结晶形成。     

Sr isotopic evidence on the spilitic degradation of the Deccan basalt

Similar Sr isotopic ratios (∼ 0.7055) for the tholeiite-spilite flow unit and the associated mineral phases, of Bombay (Deccan Traps) provide a direct evidence for the spilitic degradation of tholeiite. In contrast, a dramatic increase in the rare earth elements (REE) from basalt to spilite is rather puzzling as rare earths are considered to be relatively immobile. The geochemistry thus suggests that the process of spilitization is due to the reaction with a complex fluid having identical Sr-isotopic composition as that of the basaltic magma—thereby masking the details of the mixing process.     

IMPLICATIONS OF THE DEEP-SEATED XENOLITHS IN CENOZOIC BASALT IN KANGXIWA, WEST KUNLUN,CHINA

IMPLICATIONS OF THE DEEP-SEATED XENOLITHS IN CENOZOIC BASALT IN KANGXIWA, WEST KUNLUN,CHINA     

大兴安岭北段塔木兰沟组玄武岩地球化学及构造背景

通过对大兴安岭北段塔木兰沟组玄武岩岩石学、地球化学及其形成的大地构造环境的综合研究发现,塔木兰沟组玄武岩具有拉斑玄武岩和碱性玄武岩的双重特征,Na₂O-22O;Eu无异常,La/Yb=35.7(平均);富集Ba、Sr、K,亏损Nb.岩浆的形成可能与裂谷作用有关.在这一构造-热事件中,推测该裂谷在上黑龙江地区陆壳减薄最大,呼中地区岩石圈减薄最大,根河地区陆壳、岩石圈减薄均较小.     

冲绳海槽玄武岩中单斜辉石的特征及其意义

冲绳海槽海底玄武岩中存在着三种不同世代的单斜辉石,即斑晶,微晶和基质辉石,它们的平均粒径分别约为400μm,30μm和5μm。斑晶辉石在电子探针下可见到明显的核部与边部之分;微晶辉石无明显的环带构造;基质辉石粒径细小但数量较大。这三个世代的辉石在成分上差异较为明显,从斑晶辉石的核部→边部和微晶辉石→基质辉石,成分从富铁向富镁,钙方向演化,这与封闭体系的辉石成分演化趋势正好相反,辉石的形态和成分特征反映出该玄武岩浆存在岩浆混合的特征,它们也可反演该地幔岩浆的活动过程;源自上地幔的拉斑玄武岩浆,在从源区向浅部岩浆房上升,运移过程中,受到携带相对富铁单斜辉石的较酸性岩浆的注入,混合;在浅部岩浆房中,混合岩浆结晶形成斑晶的边部和微晶辉石;基质辉石则是岩浆喷出洋底后淬冷结晶的结果,冲 槽玄武岩中三个世代的单斜辉石较好的完整记录了地幔岩浆后系列活动过程。     

江苏近海的激流

海洋激流,尽管已在1963年就由美国WoodsHole海洋研究所海洋地质学家Hollister在旧金山召开的IUGG会议上提出,但几十年来,却因资料的缺乏而未能作进一步的论证和分析.一些国家的海洋工作者,在解释大洋海底一些沟渠的冲刷原因时,认为这是大洋海底激流所致[1～3].我国研究者修日晨在对海流异常观测记录进行分析的基础上,于1978年提出“不论在大洋或近海,均存在着一种急流”[4],讨论了海洋中存在的激流现象.