Search results for "rock"
showing 10 items of 1160 documents
Age of Palaeozoic granites and metamorphism in the Tuvino-Mongolian Massif of the Central Asian Mobile Belt: loss of a Precambrian microcontinent
2001
Abstract The Tuvino-Mongolian Massif (TMM) was previously interpreted as a Precambrian block within the Central Asian Mobile Belt. According to this idea, it consists of tectonic slices composed of metamorphic rocks of pre-Mesoproterozoic basement that experienced two episodes of regional metamorphism, and Mesoproterozoic ‘cover rocks’ that were reworked together with the basement during high-grade metamorphism. Zircon U–Pb dating of granitoids from all metamorphic complexes demonstrates that the earliest metamorphic event occurred at 536±6 Ma, significantly later than the deposition of the cover rocks. Regional upper amphibolite-facies metamorphism, which affected all metasedimentary units…
Contrasting textural record of two distinct metamorphic events of similar P-T conditions and different durations
2005
A structural, metamorphic and geochronological study of the StareMesto belt implies the existence of two distinct metamorphic events of similar peak P-T conditions (700-800 � C, 8-10 kbar) during the Cambro-Ordovician and the Carboniferous tectonometamorphic events. The hypothesis of two distinct periods of metamorphism was suggested on the basis of structural discordance between an undoubtedly Carboniferous granodiorite sill intrusion and earlier Cambro-Ordovician fabrics of a banded amphibolite complex. The analysis of crystal size distribution (CSD) shows high nucleation density (N0) and low average growth rate (Gt) for Carboniferous mylonitic metagabbros and mylonitic granodiorites. The…
Metamorphic and structural evolution of the Maures-Tanneron massif (SE Variscan chain): evidence of doming along a transpressional margin
2009
AbstractThe Variscan metamorphic and structural evolution of the Maures-Tanneron massif is divided in two main post-collisional phases: (1) a MP-MT regional gradient is developed during nappe-piling process between 350 and 320 Ma, followed by (2) LP-HT regional gradient coeval with doming between 320 and 300 Ma. During this late phase, the tectonic context was dominated by E-W shortening, which produced crustal-scale upright folds and major strike-slip displacement along trans-crustal faults. Symmetric extensional fabrics are observed on the limbs of crustal-scale anticlines, and are ascribed to local accommodation of lower crust exhumation. Heat and magma transfer are allowed by these larg…
Trace metal modeling of groundwater–gas–rock interactions in a volcanic aquifer: Mount Vesuvius, Southern Italy
2005
We report a detailed study of trace metals in groundwaters from the Somma-Vesuvius volcanic complex and present a model of the chemical processes that control the fate of these components during gas–water–rock interactions. Trace metal concentrations in Vesuvian groundwaters range from 0.01 to 0.1 Ag/l for ultra-trace elements (Sb, Cs, Co, Cd, and Pb) up to 0.1–10 mg/l for minor elements (Fe and Sr), leading to water–rock ratios from ~0.5 to 10 � 9 when normalized to trace element concentrations in the host rocks. Our results indicate non-isochemical dissolution of local volcanic rocks by groundwaters, during which mobile trace elements (As, Se, Mo, V, Li) are enriched and elements such as …
Minor and trace elements in olivines as probes into early igneous and mantle melting processes
2013
Abstract The trace element composition of olivine is a rapidly growing research area that has several applications of great potential. Mantle olivines can be distinguished from volcanic olivines by lower concentrations of Ca (
Variation of olivine composition in the volcanic rocks in the Songliao basin, NE China: lithosphere control on the origin of the K-rich intraplate ma…
2016
Abstract Lithospheric thickness and the heterogeneity of the mantle lithosphere are two major parameters that play a role in determining the final composition of the mafic melts and their minerals. The Songliao basin in northeast China represents an ideal natural laboratory to study the effect of these two parameters on early Pliocene to Holocene K-rich mafic lavas (K2O > 4 wt.%; K2O/Na2O > 1). A series of Cenozoic volcanic edifices (Erkeshan, Wudalianchi, Keluo and Xiaogulihe) are tentatively divided into three groups (Group 1 — thin, Group 2 — middle, and Group 3 — thick) according to the lithosphere thickness. They are located in the northern region of the Songliao basin extending in a n…
Lamproites as indicators of accretion and/or shallow subduction in the assembly of south-western Anatolia, Turkey
2010
IntroductionThe Western Anatolian region of Tur-key is tectonically one of the mostcomplex parts of the Alpine–Himala-yan orogenic belt (Fig. 1) because ofits long accretional palaeotectonicevolution. One of the major tectonicfeatures shaping the structure ofwestern Anatolia is the Menderesmetamorphic massif, which consistsof several assembled and imbricatedterranes exhumed during the exten-sion that affected the entire Aegeanprovince during the Late Miocene(Yilmaz et al., 2000; Okay, 2002).The Menderes Massif is usually inter-preted as a core complex delineated bylow-angle detachment faults (Bozkurtand Park, 1994; Hetzel et al., 1995;Ring et al., 2003). An alternativeexplanation associates …
Orbitally modulated black shale deposition in the upper Albian Amadeus Segment (central Italy): a multi-proxy reconstruction
2003
Abstract The upper Albian Amadeus Segment, which falls within Oceanic Anoxic Event 1c, exhibits cyclical alternations of marls/black shales and carbonate-rich beds that record evidence of orbital climate and sedimentary dynamics. A combined micropalaeontological (planktic and benthic foraminifera, and palynomorphs) and geochemical (stable isotopes, clay mineralogy, and major element distribution) investigation allowed recognition of a remarkable influence of continental material within the black shales. Moreover, the palaeoproductivity record is characterised by a non-linear response to the orbital forcing. The micropalaeontological and geochemical record suggests that upper Albian bedded c…
Zircon ages and Hf isotopic compositions of Ordovician and Carboniferous granitoids from central Inner Mongolia and their significance for early and …
2016
Abstract We present zircon ages and Hf-in-zircon isotopic data for plutonic rocks and review the evolution of central Inner Mongolia, China, in the early and late Paleozoic. Zircons of a granodiorite yielded a 206 Pb/ 238 U age of 472 ± 3 Ma that reflects the time of early Paleozoic magmatism. Zircon ages were also obtained for a tonalite (329 ± 3 Ma), quartz-diorite (320 ± 3 Ma), and granite vein (297 ± 2 Ma). Our results, in combination with published zircon ages and geochemical data, document distinct magmatic episodes in central Inner Mongolia. The dated samples are mostly granodiorite, tonalite and quartz-diorite in composition with intermediate to high-silica, high Na 2 O (3.08–4.26 w…
Stages in the compaction of peat; examples from the Stephanian and Permian of the Massif Central, France
1987
Thick coal seams in the intramontane basins of the Massif Central allow a detailed study of peat/coal compaction. Three stages of compaction can be detected. Stage 1—occurred during the deposition of the peat. Coal beds which thicken and dip away from penecontemporaneous sandstones suggest rapid compaction. Stage 2—compaction of peat related to overburden pressure in some cases resulted in the formation of lakes over peat beds. The thickness of the lacustrine deposits may give an indication of the amount of compaction that took place. One possibility is that the early compaction was related to the progradation of siliciclastics over the peat. Stage 3—occurred after the organic sediment achi…