The Datangpo‐type manganese ore deposits, which formed during the Nanhuan (Cryogenian) period and are located in northeastern Guizhou and adjacent areas, are one of the most important manganese resources in China, showing good prospecting potential. Many middle‐to‐large deposits, and even super‐large mineral deposits, have been discovered. However, the genesis of manganese ore deposits is still controversial and remains a long‐standing source of debate; there are several viewpoints including biogenesis, hydrothermal sedimentation, gravity flows, cold‐spring carbonates, etc. Geochemical data from several manganese ore deposits show that there are positive correlations between Al2O3 and TiO2, SiO2, K2O, and Na2O, and strong negative correlations between Al2O3 and CaO, MgO, and MnO in black shales and manganese ores. U, Mo, and V show distinct enrichment in black shales and inconspicuous enrichment in Mn ores. Ba and Rb show strong positive correlations with K2O in manganese ores. Cu, Ni, and Zn show clear correlations with total iron in both manganese ores and black shales. ∑REE of manganese ores has a large range with evident positive Ce anomalies and positive Eu anomalies. The Post Archean Australian Shale (PAAS) normalized rare earth element (REE) distribution patterns of manganese ores present pronounced middle rare earth element (MREE) enrichment, producing “hat‐shaped” REE plots. ∑REE of black shales is more variable compared with PAAS, and the PAAS‐normalized REE distribution patterns appear as “flat‐shaped” REE plots, lacking evident anomaly characteristics. δ13C values of carbonate in both manganese ores and the black shales show observable negative excursions. The comprehensive analysis suggests that the black shales formed in a reducing and quiet water column, while the manganese ores formed in oxic muddy seawater, which resulted from periodic transgressions. There was an oxidation–reduction cycle of manganese between the top water body and the bottom water body caused by the transgressions during the early Datangpo, which resulted in the dissolution of manganese. Through the exchange of the euphotic zone water and the bottom water, and episodic inflow of oxygenated water, the manganese in the bottom water was oxidized to Mn‐oxyhydroxides and rapidly buried along with algae. In the early diagenetic stage, Mn‐oxyhydroxides were reduced and dissolved in the anoxic pore water and then transformed into Mn‐carbonates by reacting with HCO3? from the degradation of organic matter or from seawater. In the intervals between transgressions, continuous supplies of terrigenous clastics and the high productive rates of organic matter in the euphotic zone resulted in the deposition of the black shales enriched in organic matter. 相似文献
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. 相似文献
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 TDM(Nd) values of around 2,000 Ma and zircon TDM(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. 相似文献
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. 相似文献
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. 相似文献