Tectonic environments of Cenozoic volcanic rocks in China and characteristics of the source regions in the mantle

Potassium-rich volcanism occurring throughout the Alpine-Himalayan belt from Spain to Tibet is characterized by unusually high Th/La ratios, for which several hypotheses have brought no convincing solution. Here, we combine geochemical datasets from potassic postcollisional volcanic rocks and lawsonite blueschists to explain the high Th/La. Source regions of the volcanic melts consist of imbricated packages of blueschist facies mélanges and depleted peridotites, constituting a new mantle lithosphere formed only 20 to 50 million years earlier during the accretionary convergence of small continental blocks and oceans. This takes place entirely at shallow depths (<80 km) without any deep subduction of continental materials. High Th/La in potassic rocks may indicate shallow sources in accretionary settings even where later obscured by continental collision as in Tibet. This mechanism is consistent with a temporal trend in Th/La in potassic postcollisional magmas: The high Th/La signature first becomes prominent in the Phanerozoic, when blueschists became widespread.

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Geochemical and isotopic evidence for the origin of continental flood basalts with particular reference to the Snake River Plain Idaho, U. S. A

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1984.0012 ◽

1984 ◽

Vol 310 (1514) ◽

pp. 643-660 ◽

Cited By ~ 59

Keyword(s):

Volcanic Rocks ◽

Snake River Plain ◽

Snake River ◽

Spinel Lherzolite ◽

Magma Chambers ◽

Flood Basalts ◽

Continental Flood Basalts ◽

Source Regions ◽

River Plain ◽

Crustal Magma

Voluminous outpourings of olivine and quartz tholeiite cover vast tracts of the western U.S.A, around the Columbia and Snake Rivers. Voluminous eruptive units within each province are petrographically and chemically homogeneous and generally lack significant lateral or temporal variation. These features suggest relatively homogeneous source regions. A possible scenario for the Snake River Plain involves extraction of tholeiitic melts from enriched spinel lherzolite mantle ( 87 Sr/ 86 Sr > 0.7058, 143 N d/ 144 Nd < 0.51252) which contains at least a component of 2.5 Ga material. Subsequent fractionation of olivine, plagioclase, apatite and magnetite in crustal magma chambers and simultaneous assimilation of crust ( ca . 20%) accounts for the isotopic variability in the more evolved ferrolatites and ferrobasalts. Unlike the olivine tholeiites these evolved volcanic rocks exhibit all the classic elemental and isotopic correlations consistent with an origin involving combined assimilation and fractional crystallization.

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The age and geochemistry of the mid-Cretaceous volcanic rocks in the Jinan Basin: Implications for the mid-Cretaceous tectonic environments of the Korean Peninsula and Northeast Asia

Lithos ◽

10.1016/j.lithos.2020.105383 ◽

2020 ◽

Vol 358-359 ◽

pp. 105383

Author(s):

Seung Hwan Lee ◽

Chang Whan Oh ◽

Jung-Woo Park

Keyword(s):

Korean Peninsula ◽

Volcanic Rocks ◽

Northeast Asia ◽

Tectonic Environments

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Geochemistry of volcanic rocks from the Tetagouche Group, Bathurst, New Brunswick, Canada

Canadian Journal of Earth Sciences ◽

10.1139/e78-021 ◽

1978 ◽

Vol 15 (2) ◽

pp. 207-219 ◽

Cited By ~ 8

Author(s):

R. E. S. Whitehead ◽

W. D. Goodfellow

Keyword(s):

New Brunswick ◽

Volcanic Rocks ◽

Island Arc ◽

Calc Alkaline ◽

Middle Ordovician ◽

Intermediate Range ◽

Felsic Volcanic ◽

Felsic Volcanic Rocks ◽

Tectonic Environments

The volcanic rocks of the Tetagouche Group are predominantly dacitic to rhyolitic pyroclastics and lavas; mafic alkaline and tholeiitic volcanic rocks are less abundant. Lavas representing the intermediate range (such as andesites) are uncommon.As a consequence of intense Na2O and K2O metasomatism, the mafic volcanic rocks have been classified on the basis of relatively immobile elements such as Ti, Y, Zr, Nb, Ni and Cr.By reference to volcanic suites described elsewhere for varying geologic and tectonic environments, the Tetagouche Group appears to represent two geologic environments. It is proposed that the deposition of tholeiitic and alkaline basalts accompanied the rifting associated with the opening of the Proto-Atlantic, which began during Hadrynian times. However the calc-alkaline felsic volcanic rocks were deposited on the top of the basaltic sequence along a mature island arc system that developed with the closing of the Proto-Atlantic during Middle Ordovician time.

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Proterozoic granite types in Australia: implications for lower crust composition, structure and evolution

Earth and Environmental Science Transactions of the Royal Society of Edinburgh ◽

10.1017/s0263593300007896 ◽

1992 ◽

Vol 83 (1-2) ◽

pp. 201-209 ◽

Cited By ~ 46

Author(s):

L. A. I. Wyborn ◽

D. Wyborn ◽

R. G. Warren ◽

B. J. Drummond

Keyword(s):

Volcanic Rocks ◽

Source Rocks ◽

P Wave ◽

Island Arcs ◽

Source Regions ◽

Incompatible Elements ◽

Early Palaeozoic ◽

Composition Structure ◽

Felsic Volcanic ◽

Lower Crustal

ABSTRACTGranites and their associated comagmatic felsic volcanic rocks occur in most Proterozoic provinces of Australia. Using multi-element, primordial-mantle-normalised abundance diagrams and various petrological characteristics, Australian Proterozoic granites can be subdivided into five groups: (i) I-type, Sr-depleted, Y-undepleted, restite-dominated, (ii) I- type, Sr-depleted, Y-undepleted, fractionated, low in incompatible elements, (iii) I-type Sr-depleted, Y-undepleted, enriched in incompatible elements (anorogenic granites), (iv) I-type, Sr-undepleted, Y-depleted, (v) S-type, Sr-depleted, Y-undepleted. The four Sr-depleted groups dominate, and group (iv) is of very limited extent. A comparison of these Proterozoic granites with Australian and Papua New Guinean granites of other time periods shows that these characteristic Sr-depleted Y-undepleted patterns are also dominant in early Palaeozoic granites. They are significantly different from those of granites in modern island arcs associated with subduction, and with most granites from Archaean terranes, where the multi-element diagrams are dominated by Sr-undepleted, Y-depleted patterns.The Sr-depleted, Y-undepleted patterns are thought to indicate source regions that contained plagioclase but not garnet, whilst the Sr-undepleted, Y-depleted patterns are taken to correspond with the presence of garnet, but not plagioclase, in the source rocks. The Sr-depleted, Y-undepleted patterns also only occur in regions where the lower crustal structure is dominated by an underplated mafic layer with a P-wave velocity of 7·2-7·-4 km/s. In contrast, in regions where the granites are dominated by Sr-undepleted, Y-depleted patterns, such as in the Archaean and in Cainozoic island arcs, this intermediate velocity layer is not present, and the crust-mantle boundary is very sharp.Two other distinctive compositional changes have been noted among the I-type granites of different age. Firstly, Na is highest in Archaean and Cainozoic granites, and lowest in early Proterozoic granites; Palaeozoic and Mesozoic granites have intermediate values. Secondly, late Archaean and Proterozoic granites are the most enriched in K, Th and U, while the Cainozoic and early Archaean tonalites are the most depleted; Palaeozoic and Mesozoic granites again contain intermediate amounts of those elements.

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TEM/AEM characterization of fine-grained clay minerals in very-low-grade rocks: Evaluation of contamination by empa involving celadonite family minerals

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100165938 ◽

1996 ◽

Vol 54 ◽

pp. 694-695

Author(s):

Gejing Li ◽

D. R. Peacor ◽

D. S. Coombs ◽

Y. Kawachi

Keyword(s):

Electron Microscopy ◽

Volcanic Rocks ◽

Analytical Electron Microscopy ◽

Metamorphic Rocks ◽

New Mineral ◽

Chemical Compositions ◽

Low Grade ◽

Fine Grained ◽

Depth Analysis ◽

Mineral Aggregates

Recent advances in transmission electron microscopy (TEM) and analytical electron microscopy (AEM) have led to many new insights into the structural and chemical characteristics of very finegrained, optically homogeneous mineral aggregates in sedimentary and very low-grade metamorphic rocks. Chemical compositions obtained by electron microprobe analysis (EMPA) on such materials have been shown by TEM/AEM to result from beam overlap on contaminant phases on a scale below resolution of EMPA, which in turn can lead to errors in interpretation and determination of formation conditions. Here we present an in-depth analysis of the relation between AEM and EMPA data, which leads also to the definition of new mineral phases, and demonstrate the resolution power of AEM relative to EMPA in investigations of very fine-grained mineral aggregates in sedimentary and very low-grade metamorphic rocks.Celadonite, having end-member composition KMgFe3+Si4O10(OH)2, and with minor substitution of Fe2+ for Mg and Al for Fe3+ on octahedral sites, is a fine-grained mica widespread in volcanic rocks and volcaniclastic sediments which have undergone low-temperature alteration in the oceanic crust and in burial metamorphic sequences.

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