Origin of composite dikes in the Gouldsboro granite, coastal Maine

Composite dikes, consisting of aphyric basaltic margins and phenocryst-rich rhyolitic interiors, cut the Gouldsboro granite of coastal Maine at many localities. Limited hybridization (exchange of crystals, commingling, and mixing) occurs in most of the dikes and indicates that the two magmas were contemporaneous with emplacement of rhyolitic magma following closely in time the initial emplacement of the basaltic dike. Petrographic characteristics and geochemistry indicate that the source of the rhyolite was resident magma in the Gouldsboro granite magma chamber. The composite dikes formed when basaltic dikes ruptured the Gouldsboro magma chamber, permitting partly crystallized magma from the margin of the chamber to flow outward into the center of the basaltic dikes. Field relations of similar composite dikes in other areas (e.g., Iceland, Scotland) are consistent with this model. A second type of composite dike (silicic margins with chilled basaltic pillows) commonly cuts mafic intrusions along the Maine coast and probably formed when a granitic dike ruptured an established chamber of mafic magma, permitting resident mafic magma to collapse downward into the still Liquid granitic dike. Most composite dikes have probably formed when a magma chamber was disrupted by a dike of contrasting magma rather than by tapping a stratified magma chamber.     

云南临沧花岗岩的冲断叠瓦构造与推覆构造

 云南省西部沿澜沧江分布的临沧花岗岩,呈SN向延伸,长达500km,但平均宽度只有25km,系逆冲与推覆叠置变形缩短的结果。岩片冲断和推覆的方向普遍为自西向东,临沧花岗岩带向东推覆的距离为30-80km,最大距离120km,冲断叠瓦构造和推覆构造形成的时代主要为中、新生代。糜棱岩的同位素年龄为15.43Ma、25.55Ma和179Ma.新生代沿冲断层发生了近SN向水平走滑运动和沿NE、NW向断层的剪切运动。     

临沧锗矿床的稀土元素地球化学研究

通过对临沧锗矿床中基底花岗岩、矿化煤以及硅质岩的稀土元素地球化学研究认为盆地基底花岗提供了形成锗矿化的锗和形成硅质岩的硅；本区陆相热水成因硅质岩的Ce异常不明显，相对富集LREE；矿化煤相对富集HREE，这可能主要与矿化煤在含锗热液的相互作用过程中，煤对重烯土的优先富集有关。     

Granite fracture index to check suitability of granite outcrops for quarrying

A correct knowledge of fracture patterns previous to quarrying is important for economical exploitation. This work comprises the general characterization of the discontinuities of 10 granite rock masses in NE Portugal. During the study some discontinuity characteristics such as orientation, persistence, aperture, trace length and spacing were evaluated. The differences in the characteristics of the selected granites were described, and the most important exploration factors were identified. In the granites studied, most of the joints are oriented from N10°E to N50°E, which is compatible with the regional fracture pattern. Concerning joint termination, the XX type is common, while the RR type is rare; the persistence index ranges from 3.5% to 25.4%. Closed joints are predominant in the granites studied (> 72.4%), and openings of the others are about 1–2 mm as a result of superficial displacement of the granite blocks. The percentage of filled joints, usually by quartz and pegmatite, is different from one granite to another, but most are < 20%. The average joint length ranges from 11.1 m in post-tectonics granites to 4.6 m in syntectonic granites. Joint spacing varies from 0.98 to 2.8 m with higher values in post-tectonic granites. Joint-space distributions of the granites studied are close to log-normal and exponential distributions. Volumetric joint count (Jv) ranges from 0.8 to 2.1 joints/m³ and is used for categorizing the places of each granite. The relationships between Jv and other joint characteristics, such as length, spacing and number of joint sets, were investigated. A Granite Fracturing Index (GFI) is introduced, computed on the basis of volumetric joint count, number of joint sets and true joint spacing. The GFI classification of studied places allows us evaluate the suitability for profitable exploitation.     

Evidence for the thrust emplacement of the ‘Lesser Himalaya’ Chur granite,Himachal Pradesh

Numerous peraluminous and porphyritic granitic bodies and augen gneisses of granitic compositions occur in the nappe sequences of the Lower Himalaya. They are Proterozoic-to-lower Paleozoic in age and have been grouped into the ‘Lesser Himalaya granite belt’. The mode of emplacement and tectonic significance of these granites are as yet uncertain but they are generally considered to be sheet-like intrusions into the surrounding rocks. The small and isolated granite body (the Chur granite) that crops out around the Chur peak in the Himachal Himalaya is one of the more famous of these granites. Several lines of evidence have been adduced to show that the Chur granite has a thrust (the Chur thrust) contact with the underlying metasedimentary sequence (locally called the Jutogh Group). The Chur granite with restricted occurrence at the highest topographic and structural levels represents an erosional remnant of a much larger sub-horizontal thrust sheet. The contact relations between the country rocks and many of the other granite and granitic augen gneisses in the Lesser Himalaya belt are apparently similar to that of the Chur granite suggesting that at least some of them may also represent thrust sheets.     

Zircon age and Nd–Hf isotopic composition of the Yunnan Tethyan belt,southwestern China

The Baoshan block of the Tethyan Yunnan, southwestern China, is considered as northern part of the Sibumasu microcontinent. Basement of this block that comprises presumably greenschist-facies Neoproterozoic metamorphic rocks is covered by Paleozoic to Mesozoic low-grade metamorphic sedimentary rocks. This study presents zircon ages and Nd–Hf isotopic composition of granites generated from crustal reworking to reveal geochemical feature of the underlying basement. Dating results obtained using the single zircon U–Pb isotopic dilution method show that granites exposed in the study area formed in early Paleozoic (about 470 Ma; Pingdajie granite) and in late Yanshanian (about 78–61 Ma, Late Cretaceous to Early Tertiary; Huataolin granite). The early Paleozoic granite contains Archean to Mesoproterozoic inherited zircons and the late Yanshanian granite contains late Proterozoic to early Paleozoic zircon cores. Both granites have similar geochemical and Nd–Hf isotopic charateristics, indicating similar magma sources. They have whole-rock T _DM(Nd) values of around 2,000 Ma and zircon T _DM(Hf) values clustering around 1,900–1,800 and 1,600–1,400 Ma. The Nd–Hf isotopic data imply Paleoproterozoic to Mesoproterozoic crustal material as the major components of the underlying basement, being consistent with a derivation from Archean and Paleoproterozoic terrains of India or NW Australia. Both granites formed in two different tectonic events similarly originated from intra-crustal reworking. Temporally, the late Yanshanian magmatism is probably related to the closure of the Neotethys ocean. The early Paleozoic magmatism traced in the Baoshan block indicates a comparable history of the basements during early Paleozoic between the SE Asia and the western Tethyan belt, such as the basement outcrops in the Alpine belt and probably in the European Variscides that are considered as continental blocks drifting from Gondwana prior to or simultaneously with those of the SE Asia.     

Fluid and melt inclusion evidence for the origin of idiomorphic quartz crystals in topaz-bearing granite from the Salmi batholith, Karelia, Russia

Idiomorphic quartz crystals in topaz-bearing granite from the Salmi batholith contain primary inclusions of silicate melt and abundant mostly secondary aqueous fluid inclusions. Microthermometric measurements on melt inclusions give estimates for the granite solidus and liquidus of 640–680°C and 770–830°C, respectively. Using published solubility models for H₂O in granitic melts and the obtained solidus/liquidus temperatures from melt inclusions, the initial water concentration of the magma is deduced to have been approximately 3 wt.% and the minimum pressure about 2 kbar. At this initial stage, volatile-undersaturation conditions of magma were assumed. These results indicate that the idiomorphic quartz crystals are magmatic in origin and thus real phenocrysts. During subsolidus cooling and fracturing of the granite, several generations of aqueous fluid inclusions were trapped into the quartz phenocrysts. The H₂O inclusions have salinities and densities of 1–41 wt.% NaCl eq. and 0.53–1.18 g/cm³, respectively.     

Highly fractionated I-type granites in NE China (II): isotopic geochemistry and implications for crustal growth in the Phanerozoic

NE China is the easternmost part of the Central Asian Orogenic Belt (CAOB). The area is distinguished by widespread occurrence of Phanerozoic granitic rocks. In the companion paper (Part I), we established the Jurassic ages (184–137 Ma) for three granitic plutons: Xinhuatun, Lamashan and Yiershi. We also used geochemical data to argue that these rocks are highly fractionated I-type granites. In this paper, we present Sr–Nd–O isotope data of the three plutons and 32 additional samples to delineate the nature of their source, to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the Phanerozoic.

Despite their difference in emplacement age, Sr–Nd isotopic analyses reveal that these Jurassic granites have common isotopic characteristics. They all have low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7045±0.0015), positive _Nd(T) values (+1.3 to +2.8), and young Sm–Nd model ages (720–840 Ma). These characteristics are indicative of juvenile nature for these granites. Other Late Paleozoic to Mesozoic granites in this region also show the same features. Sr–Nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two-stage processes: (1) formation of parental magmas by melting of a relatively juvenile crust, which is probably a mixed lithology formed by pre-existing lower crust intruded or underplated by mantle-derived basaltic magma, and (2) extensive magmatic differentiation of the parental magmas in a slow cooling environment.

The widespread distribution of juvenile granitoids in NE China indicates a massive transfer of mantle material to the crust in a post-orogenic tectonic setting. Several recent studies have documented that juvenile granitoids of Paleozoic to Mesozoic ages are ubiquitous in the Central Asian Orogenic Belt, hence suggesting a significant growth of the continental crust in the Phanerozoic.     

滇西“三江”地区临沧花岗岩基中三叠世碱长花岗岩的发现及其意义

在野外调研的基础上,对滇西“三江”地区临沧花岗岩基中勐库碱长花岗岩体的年代学、岩石学和地球化学开展了较系统的研究。结果显示,勐库岩体的锆石LA-ICP-MS U-Pb年龄为236.2±3.7 Ma,表明岩体应形成于中三叠世,而非前人认为的新生代。全岩主、微量元素,Sr同位素和锆石微量元素特征显示岩体属于S型花岗岩系列,来自于与临沧花岗岩基相类似的地壳沉积物源区的部分熔融,并经历了高程度的结晶分异作用。以勐库岩体为代表的碱长花岗岩与临沧花岗岩基形成时代一致,表明碱长花岗岩应为临沧花岗岩基的重要组成部分。除已报道的黑云母二长花岗岩类和黑云母花岗闪长岩类外,临沧花岗岩基还应包括较广泛分布的碱长花岗岩类。本文同时揭示,由于临沧花岗岩基中以勐库岩体为代表的碱长花岗岩并非早前认为的新生代岩体,“三江”南段特别是临沧地块前人划分的新生代岩体分布范围和规模可能需要重估。     

马达加斯加Maevatanana金矿区花岗岩锆石U-Pb年龄、Hf同位素特征及其地质意义

马达加斯加中部Maevatanana、Andriamena和Beforona三条绿岩带共有上百处铬、镍、铁、金等金属矿点。对位于马达加斯加中北部绿岩带的Maevatanana西南部花岗岩体进行主量元素、微量元素、锆石LA-MC-ICP-MS U-Pb定年及Lu-Hf同位素分析。岩体主要由黑云母二长花岗岩构成,为准铝质到弱过铝质的高钾钙碱性系列花岗岩,富集Rb、Ba、K等大离子亲石元素(LILE),亏损Nb、Ta、Ti等高场强元素(HFSE),其稀土元素总量为∑REE=92.02×10-6,A/CNK=1.01。其内锆石LA-MCICP-MS U-Pb的年龄为739~767Ma,即中新元古代,其εHf(t)均为负值,揭示该矿区花岗岩体源区为陆壳。其中岩浆锆石和继承锆石的Hf同位素二阶段模式年龄相近,表明花岗岩源岩的形成时期和基底黑云角闪斜长片麻岩的原岩一致。地球化学性质显示花岗岩体为I型花岗岩,与埃达克岩类似。综合数据认为岩体形成于中-新元古代(824~720Ma)Rodinia超大陆裂解,伴随Mozambique洋闭合Tanzanian克拉通与印度Dharwar克拉通汇聚背景下的陆缘弧岩浆活动。