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Magmatism of Phanerozoic Fold Belts
Published in O.A. Bogatikov, R.F. Fursenko, G.V. Lazareva, E.A. Miloradovskaya, A. Ya, R.E. Sorkina, Magmatism and Geodynamics Terrestrial Magmatism Throughout the Earth’s History, 2020
V.V. Yarmolyuk, V.I. Kovalenko
The overridden spreading zone or “asthenospheric window” seems to have paused beneath Khangai during the late Permian. It may have been related to the collision of the North Asian and Sino-Korean late Palaeozoic palaeocontinents, which resulted in the closure of the late Palaeozoic Palaeotethys, at least in its western part. This process resulted in the formation of a new zonally symmetrical type late Permian magmatic zone (Fig. 3.11). As mentioned above, its core consisted of granitoids of the Khangai Batholith formed at the site of the Khangai Trough. The rocks of the trough are assumed to have been molten, forming large volumes of granitoid melts due to deep-seated asthenospheric heat sources, including mantle magmas reaching the crust (Kovalenko and Yarmolyuk, 1990). Rift zones with alkaline bimodal magmatism are characteristic of the peripheral parts of the area. Apparently, the extensive midland marine basin was also initially a rift system, marked by poorly developed, and, as yet, poorly known magmatism with island-arc characteristics, which can be traced in the eastern part of the axial zone of the area (Djargalantuyn and North Gobi marine basins).
Plutonic Rocks
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
El Capitan is mostly composed of light gray granite or rocks closely related to granite. Collectively termed granitic, or sometimes granitoid, these light-colored rocks are composed chiefly of feldspars and quartz. But, if you focus in, you can see different colors of rock exposed in El Capitan’s face, suggesting that there may be significant compositional variation. In places, a much darker rock, dark gray to black, is seen in blobs and splotches. This rock becomes especially prominent as you drive farther into the Valley, around the Nose, and get a view of El Capitan’s southeast face (seen in Fig. 6.2), where a large black body of rock is exposed in the North American Wall, so named because the body of rock is shaped like the continent. On both the southwest and southeast faces, a myriad of white to light gray dikes—small veins up to several meters thick—cut across the face in near-horizontal directions.
Has the tectonic regime of the Baltic Shield always remained the same?
Published in GFF, 2022
where T(z) is the temperature as a function of depth, z (positive downwards), A is the radiogenic heat-production, K is the thermal conductivity, D is the continental crustal thickness, and Q is the mantle heat flow. At 1.8 Ga, this simple model gives temperatures in the lower parts of a hypothetical, homogeneous crust (thickness 40 km) well above the solidus of granitic rocks (Fig. 2). Figure 2 suggests that at c. 1.8 Ga or earlier, this crust starts melting in its lower part, to advect heat to shallower levels by magma transport and to give rise to an upper crust enriched in granitoids. Incompatible, radioactive elements are allocated to the magma resulting in an impoverished magma-provenance area. A recent, also hypothetical, homogeneous crust could remain stable.
Character and tectonic setting of plutonic rocks in the Gällivare area, northern Norrbotten, Sweden
Published in GFF, 2019
Zmar Sarlus, Olof Martinsson, Tobias E. Bauer, Christina Wanhainen, Joel B. H. Andersson, Roger Nordin
Classification of tectonic setting for felsic rocks and especially those of plutonic origin is complicated by the nature of felsic magma which usually is formed by several interacting processes and with different melt sources (Winter 2001). However, combining various classification schemes, such as those proposed by Pearce et al. (1984), Harris et al. (1986) and the S-I-A-M classification by Chappell & White (1974), White (1979) and Winter (2001) can provide some confidence in identifying tectonic settings of granite and syenite. According to various tectonic classification diagrams (Fig. 8B–D) the granites and syenites from the Gällivare area could have formed in an orogenic (continental arc) to transitional (in-between orogenic and anorogenic, such as post-orogenic related to uplift and collapse) setting using the nomenclature of Winter (2001). This conforms to their calc-alkaline to shoshonitic, metaluminous to weakly peraluminous, dominantly I-type character. In a continental arc setting, felsic magma is the product of mantle wedge melting due to fluids and dissolved components released from the subducting slab followed by fractionation during magma migration in the crust which generates I-type calc-alkaline granitoids (Winter 2001). In transitional settings, the origin of magma and melting mechanism is attributed to heat from rising asthenosphere and mantle magmas causing partial melting of lower and mid-crust generating A- to S-type granitoids (Winter 2001). Based on the limited number of samples from the Gällivare area and due to the difficulties in interpreting tectonic setting of felsic rocks based on trace element data, either of these two environments, volcanic-arc or transitional setting may be an option.
Petrogenesis of Jurassic Xietongmen intrusive rocks at the southern margin of the Lhasa terrane: implications for intra-oceanic arc evolution
Published in Australian Journal of Earth Sciences, 2020
Generally, felsic magmas cannot be directly generated by partial melting of the mantle (Taylor & McLennan, 1985), as very low-degree hydrous melting of peridotite could produce andesitic melts at most. Felsic magmas are thought to form during two-stage processes where juvenile crustal material forms by the underplating of mantle-derived magma at the base of the crust. This juvenile crustal material then undergoes melting during replenishment of the underplating magma by new mafic melts. The granitoids in the present study area have similar εHf(t) isotopic compositions to the gabbroic units in this region, suggesting that the two are genetically related. This in turn suggests that the gabbroic rocks are likely to represent solidified underplated basaltic magma whereas the granitoids maybe the result of the melting, assimilation, storage, and homogenisation (MASH) processes between these underplated basaltic magmas and melts derived from overlying crustal material (i.e. juvenile crustal material; Meng, Dong et al., 2016; Meng, Xu et al.,2016; Qiu et al., 2015). The MASH model (Hildreth & Moorbath, 1988) provides a clear explanation of the genesis of the hornblende gabbro–granodiorite complexes in the study area as well as the abundant MMEs within the granodiorites. This model describes the generation of hot, hydrous, relatively oxidised, and sulfur-rich mafic magmas (predominantly basalts) from a metasomatised region of the mantle wedge above a subducting oceanic slab. These melts ascend to the base of the overlying crust where they stall as a result of density contrasts with overlying material and begin to crystallise, releasing heat that causes the partial melting of the overlying crustal rocks that generated the granodiorites in the study. The interaction of these underplating melts with remelted juvenile lower crustal material also generated more silica-rich magmas as well as mafic enclaves (e.g. MMEs within the granodiorites in the study) generated by binary mixing between gabbroic and granitic magmas.