subduction setting
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Minerals ◽  
2022 ◽  
Vol 12 (1) ◽  
pp. 64
Author(s):  
Micol Bussolesi ◽  
Giovanni Grieco ◽  
Alessandro Cavallo ◽  
Federica Zaccarini

Mg-Fe2+ diffusion patterns in olivine and chromite are useful tools for the study of the thermal history of ultramafic massifs. In the present contribution, we applied the exponential modeling of diffusion patterns to geothermometry and geospeedometry of chromitite ores from two different ophiolite contexts. The Iballe ophiolite (Northern Albania) hosts several chromitite pods within dunites. Primary and re-equilibrated Mg#, estimated by using an exponential function, provided re-equilibration and primary temperatures ranging between 677 and 996 °C for chromitites and between 527 and 806 °C for dunites. Cooling rates for chromitites are higher than for dunites, suggesting a different genesis for the two lithologies, confirmed also by spinel mineral chemistry. Chromitites with MORB affinity formed in a SSZ setting at a proto-forearc early stage, explaining the higher cooling rates, while dunites, with boninitic affinity, were formed deeper in the mantle in a more mature subduction setting. At the Nea Roda ophiolite (Northern Greece) olivine in chromitites do not show Mg-Fe variations, and transformation into ferrian chromite produced “fake” diffusion patterns within chromite. The absence of diffusion patterns and the low estimated temperatures (550–656 °C) suggest that Nea Roda chromitites were completely re-equilibrated during an amphibolite-facies metamorphic event that obliterated all primary features.


Author(s):  
Joventine Decol ◽  
Monica Heilbron ◽  
Caroline Peixoto ◽  
Henrique Bruno ◽  
Samuel Moreira Bersan ◽  
...  
Keyword(s):  

Lithosphere ◽  
2021 ◽  
Vol 2021 (1) ◽  
Author(s):  
Shao-Wei Zhao ◽  
Chao Yang ◽  
Shao-Cong Lai ◽  
Xian-Zhi Pei ◽  
Zuo-Chen Li ◽  
...  

Abstract The petrogenesis and evolution process of continental arc magmatism provide insight into discovering the formation and differentiation of continental crust. Therefore, the geochemical, isotopic, and mineralogical analyses were conducted for coeval continental arc igneous rocks in the Tengchong Block to clarify their evolution process in the continental arc magmatic systems. The Middle Triassic appinites in the Tengchong Block, southeastern extension of Tibet, were generated at the subduction setting with zircon U-Pb age of ca. 243 Ma. The Nb/Yb, Zr/Yb, and Ta/Yb ratios along with depleted zircon Hf isotopic compositions indicate a source with an N-MORB affinity for the appinites. However, relatively enriched whole-rock Sr-Nd isotopic compositions with the characteristic of high Sr/Nd, Ba/Th, Th/La, and Th/Nd ratios suggest the source was metasomatized by ~2% subducted sediment-derived fluid. According to the REE ratios modeling, the primary magma of Nabang appinites was due to 5-10% partial melting of such metasomatized mantle source. The appinites are characterized by variable compositions, such as SiO2 contents of 47.82-61.74 wt.% and MgO of 10.61-2.61 wt.%, which resulted from the polybaric and multistage fractional crystallization of a slightly hydrous primary magma in a thick crust. At lower crustal pressures, clinopyroxene was the main fractionating phase, and at middle crustal pressures, amphibole+magnetite were the dominant fractionating phases; predominant plagioclase fractionation occurred at the magma emplacement level. This process could be an effective mechanism to induce the differentiation of continental crust. The fractionation of clinopyroxene and amphibole, accompanied by suppressing plagioclase at lower-middle crustal pressures, induces the high alumina in the evolved melt and forms high-alumina basaltic to andesitic magma.


Lithos ◽  
2021 ◽  
Vol 382-383 ◽  
pp. 105949
Author(s):  
Nicole Dilissen ◽  
Károly Hidas ◽  
Carlos J. Garrido ◽  
Vicente López Sánchez-Vizcaíno ◽  
Wolf-Achim Kahl

2021 ◽  
Vol 48 (1) ◽  
pp. 24
Author(s):  
Denisse De la Fuente ◽  
Óscar Figueroa ◽  
Daniel Demaiffe ◽  
Mauricio Mella ◽  
Paul Duhart ◽  
...  

Upper Cretaceous intrusives of limited extent crop out in the Coastal Cordillera near of Valdivia (39º48’ S), 100 km west of the main topographic divide of the Andean Cordillera. Given that plutonic rocks of the same age crop out at the same latitudes in the high Andes the coastal intrusives emplaced in a forearc position in the upper plate of a subduction setting. They correspond to hypabyssal intrusives displaying mainly porphyritic texture and lithological variations with microtonalites (minor), porphyritic microgranodiorites (main) and microgranites. They intrude the Upper Paleozoic-Triassic accretionary complex of the Bahia Mansa Metamorphic Complex. These intrusives, that comprise the Chaihuín Pluton and minor stocks of porphyritic felsic rocks, have calc-alkaline affinities with metaluminous and peraluminous character. They are geochemically similar to the contemporaneous main arc-related plutonic rocks of the Gualletué Plutonic Group. The microgranitoids and dacitic rocks from Los Boldos, the low and Loncoche are peculiar because they show an apparently adakitic affinity in Sr/Y and LaN/YbN discriminant diagrams; nevertheless Sr contents of these rocks (


2020 ◽  
Vol 123 (4) ◽  
pp. 633-648
Author(s):  
D.H. Cornell ◽  
M. Harris ◽  
B.S. Mapani ◽  
T. Malobela ◽  
D. Frei ◽  
...  

Abstract The volcanosedimentary Guperas Formation contains the youngest volcanic rocks of the Sinclair Supergroup in the Konkiep Terrane of southern Namibia. Precise U-Pb zircon microbeam dating shows that the Guperas Formation as mapped includes felsic volcanic rocks which belong to both the first (1.37 to 1.33 Ga) and the third (1.11 to 1.07 Ga) magmatic cycle of the Sinclair Supergroup. Volcanic rocks of the ‘true’ Guperas Formation are dated by three samples, with a combined age of 1108 ± 10 Ma. The sedimentary rocks mapped as Guperas Formation are also distinguished by two different detrital age spectra into the ~1 100 Ma true Guperas Formation and the Aruab Member of the ~1 217 Ma Barby Formation. Geochronology now resolves the previous stratigraphic separation of the very similar Nubib and Rooiberg (Sonntag) Granites. The two small outcrops of 1 334 ± 5 Ma Rooiberg Granite are now shown to be part of the regional 1 334 ± 8 Ma Nubib Granite batholith. The Konkiep Terrane was affected by faulting and shear zones, but was only gently folded and not involved in regional metamorphism, despite its proximity to the Namaqua-Natal Province to the southwest. This is due to the Konkiep Terrane having a thick and strong continental basement which may have formed as part of the mainly Palaeoproterozoic Rehoboth Province. However no Palaeoproterozoic rocks are exposed in the Konkiep Terrane, which is now interpreted as an unaffiliated terrane. The three cycles of extrusive and plutonic magmatism in the Sinclair Supergroup formed in chronologically distinct periods and different tectonic settings, which requires revision of the stratigraphic nomenclature. The Konkiep Group is replaced by three new groups which are separated by >100 million-year unconformities. The Betta Group, represented by the mainly volcanic Kumbis, Nagatis and Welverdiend formations in the first magmatic cycle, probably formed in a passive continental rift setting due to breakup of the Rehoboth Province between 1 374 and 1 334 Ma. The Vergenoeg Group, represented by the sedimentary Kunjas and volcanic Barby and Haiber Flats formations, formed in a subduction setting at the margin of the Konkiep Terrane. This ~1 217 to 1204 Ma magmatic cycle ended with the accretion of Namaqua-Natal terranes to the Kalahari Craton. The ~1 100 Ma Ganaams Group, represented by the volcanic Guperas Formation and sedimentary Aubures Formation, was the result of interplay between the continental-scale Umkondo mantle heating event and movements between crustal blocks following the Namaqua-Natal collisional orogeny.


Minerals ◽  
2020 ◽  
Vol 10 (9) ◽  
pp. 799
Author(s):  
Chao Zhang ◽  
Franz Neubauer ◽  
Zheng-Hong Liu ◽  
Fang-Hua Cui ◽  
Qing-Bin Guan

This paper reports new zircon LA–ICP–MS U–Pb and Hf isotope data, and whole-rock major and trace element data for Late Permian to Early Triassic intrusive rocks in the Yanbian area, NE China. These data provide new insights into the timing of the final subduction of the Paleo-Asian Ocean beneath the North China Craton. The zircon U–Pb age data indicate that a suite of Late Permian to Early Triassic intrusive rocks related to subduction is present within the Yanbian area. The Late Permian intrusive rocks consist of diorites while the Early Triassic granites and hornblende gabbros constitute a geochemically bimodal igneous rock association. Furthermore, the Early Triassic granites show the geochemical characteristics of shoshonitic rocks. All the rocks are characterized by enrichment in LILEs and LREEs, and depletion in HREEs and HFSEs, suggesting they formed in a subduction setting. Zircons from the Early Triassic gabbros have εHf(t) values and TDM2 ages of +7.6 to +10.7 and 735–1022 Ma, respectively, suggesting that they formed from a primary magma generated by the partial melting of lithospheric mantle material that had been previously modified by subduction-related fluids. The Late Permian diorites have εHf(t) values and TDM2 ages of +0.5 to +9.5 and 853 to 1669 Ma, respectively, while they have high contents of Al2O3, Fe2O3, and low contents of SiO2, Cr, and Ni, indicating Late Permian diorites should derive from the mantle and are influenced by some crustal material. Early Triassic granitic rocks have a wide range of εHf(t) values and TDM2 ages of −4.8 to +9.4 and 852 to 2136 Ma, respectively. Their zircons imply that the Early Triassic granites could be mainly derived from partial melting of the crust, with minor contribution of the crustal material of an ancient crust. The Early Triassic bimodal intrusive rocks in Yanbian area, combined with the regional geologic information; therefore, record a final post-subduction extensional environment due to the break-off of the previously subducted slab.


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