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The tectonic and crustal evolution of the East Kunlun Orogen in the Triassic remains unclear. We investigated the Triassic andesites (235 and 216 Ma) to investigate these issues. The Mid- and Late Triassic andesites are calc-alkaline with enriched Sr–Nd isotopic compositions: (87Sr/86Sr)i = 0.709210–0.709758 and ε Nd(t) = −8.0 to −6.9. The Mid-Triassic andesites have higher Mg, Cr and Ni contents and more depleted Hf isotopic compositions (ε Hf(t) = −4.2 to 1.3) than the Late Triassic andesites (ε Hf(t) = −7.4 to −5.8). These features are explained by the derivation of magma from partial melting of the mafic lower continental crust, although minor lithospheric mantle materials are also involved in the Mid-Triassic andesites. The andesites were generated in extensional settings related to slab break-off in the Mid-Triassic and lithospheric delamination in the Late Triassic, respectively. The Late Triassic magmatic suites are higher in K and represent a more mature continental crust, indicating that crustal reworking in extensional settings contributes to crustal maturation. In combination with the regional magmatic, structural and sedimentary records, we propose that the East Kunlun Orogen evolved from oceanic subduction in the Late Permian to Mid-Triassic through continental collision in the Mid-Triassic to post-collision in the Late Triassic. [ABSTRACT FROM AUTHOR]

Scottish 'Newer Granites' record the evolution of the Caledonides resulting from Iapetus subduction and slab breakoff during the Silurian–Devonian Scandian Orogeny, but relationships between geodynamics, petrogenesis and emplacement are incomplete. Laser ablation U–Pb results from magmatic zircons at the Cluanie Pluton (Northern Highlands) identify clusters of concordant Silurian data points. A cluster with a weighted mean 206Pb/238U age of 431.6 ± 1.3 Ma (2 σ confidence interval, n = 6) records emplacement whilst older points (clustered at 441.8 ± 2.3 Ma, n = 9) record deep crustal hot zone magmatism prior to ascent. The Cluanie Pluton, and its neighbour the c. 428 Ma Clunes tonalite, have adakite-like high Na, Sr/Y, La/Yb and low Mg, Ni and Cr characteristics, and lack mafic facies common in other 'Newer Granites'. These geochemical signatures indicate the tapping of batches of homogenized, evolved magma from the deeper crust. The emplacement age of the Cluanie Pluton confirms volumetrically modest subduction-related magmatism occurred beneath the Northern Highlands before slab breakoff, probably restricted by crustal thickening during the c. 450 Ma Grampian 2 event. Extensive new in situ geochemical–geochronological studies for this terrane may further substantiate the deep crustal hot zone model and the association between Caledonian magmatism and potentially metallogenesis. The term 'Newer Granites' is outdated as it ignores geochronology, petrography and tectonic setting. Hence, 'Caledonian intrusions' would be a more appropriate generic term to cover those bodies related to either Iapetus subduction or to slab breakoff. Supplementary material: Full geochronological, whole rock geochemical and geobarometric data are available at https://doi.org/10.6084/m9.figshare.c.6305927 [ABSTRACT FROM AUTHOR]

Adakitic rocks occur in a variety of tectonic settings and are key to understanding the tectonic evolution and geodynamics of orogenic belts. We investigated latest Oligocene (23.5–22.5 Ma) quartz monzonites and granites from the western segment of the Urumieh–Dokhtar magmatic belt in Iran, which are likely to have formed in response to the early stages of Arabia–Eurasia collision. The studied rocks have the geochemical characteristics of typical adakites, such as high SiO2 (60.18–68.82 wt%) and Sr (499–793 ppm) contents, low Y (8.90–17.1 ppm) and Yb (0.88–1.58 ppm) contents, and high Sr/Y (26.1‒67.8) and (La/Yb)N (21.9‒32.9) ratios. They have variable K2O (3.88–5.09 wt%), MgO (0.44–2.74 wt%; Mg# = 33.7–52.5), Cr (4.27–40.59 ppm), Ni (4.28–35.68 ppm) and Th (9.56–59.59 ppm) contents, and relatively depleted Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.70450–0.70516; ε Nd(t) = 2.1–2.7]. These characteristics indicate that the quartz monzonites were derived from the partial melting of delaminated lower crust that interacted with mantle peridotite with high MgO, Cr and Ni contents and depleted Sr–Nd isotopic compositions and suggest that the granites were formed by the fractional crystallization of quartz monzonitic magma. The geochemical features of the studied adakitic rocks could therefore have been affected by magmatic processes (e.g. fractional crystallization), which might be misleading in interpretations of their petrogenesis and related tectonic settings. The geochemical features of the studied rocks indicate that the crust of the western segment of the Urumieh–Dokhtar magmatic belt was thickened to c. 50 ± 4.43 km during the latest Oligocene (c. 23.5 Ma) as a result of Arabia–Eurasia collision. Supplementary material: Whole rock geochemical and Sr–Nd isotopic compositions, and zircon U–Pb and Lu–Hf isotopic data are available at https://doi.org/10.6084/m9.figshare.c.6188328 [ABSTRACT FROM AUTHOR]

Recent empirical calibrations of Sr/Y and La/Yb from intermediate igneous rocks as proxies of crustal thickness yield discrepancies when applied to high ratios from thick crust. We recalibrated Sr/Y and La/Yb as proxies of crustal thickness and applied them to the Gangdese Mountains in southern Tibet. Crustal thickness at 180-170 Ma decreased from 36 to 30 km, consistent with Jurassic backarc extension and ophiolite formation along the southern Asian margin during Neo-Tethys slab rollback. Available data preclude detailed estimates between 170 and 100 Ma and tentatively suggest ~55 km thick crust at ca. 135 Ma. Crustal thinning between 90 and 65 Ma is consistent with a phase of Neo-Tethys slab roll-back that rifted a portion of the southern Gangdese arc (the Xigaze arc) from the southern Asian margin. Following the continental collision between India and Asia, crustal thickness increased by ~40 km at ~1.3 mm/a between 60 and 30 Ma to near modern crustal thickness, before the onset of Miocene east-west extension. Sustained thick crust in the Neogene suggests the onset and later acceleration of extension in southern Tibet together with ductile lower crustal flow works to balance the ongoing mass addition of under-thrusting Indian crust and maintain isostatic equilibrium. [ABSTRACT FROM AUTHOR]

The Yellowstone hotspot is recognized as a whole-mantle plume with a history that extends to at least 56 Ma, as recorded by offshore volcanism on the Siletzia oceanic plateau. Siletzia accreted onto the North American plate at 51-49 Ma, followed by repositioning of the Farallon trench west of Siletzia from 48 to 45 Ma. North America overrode the hotspot, and it transitioned from the Farallon plate to the North American plate from 42 to 34 Ma. Since that time, it has been genetically associated with a series of aligned volcanic provinces associated with age- progressive events that include Oligocene high-K calc-alkaline volcanism in the Oregon backarc region with coeval adakite volcanism localized above the hot plume center; mid- Miocene bimodal and flood-basalt volcanism of the main-phase Columbia River Basalt Group; coeval collapse of the Nevadaplano associated with onset of Basin and Range extension and minor magmatism; and late Miocene to recent bimodal volcanism along two coeval but antithetical rhyolite migration trends-the Yellowstone-Snake River Plain hotspot track to the ENE and the Oregon High Lava Plains to the WNW. [ABSTRACT FROM AUTHOR]

The Olkhovsky ring complex (ORC) is composed of a two-phase magmatic rock association with quartz dioritequartz monzodiorite-granodiorite suite in the outer ring (OR) and trondhjemite as the inner stock (IS). Zircon U-Pb dating reveals that the rocks of the OR and IS were emplaced at 2070 ± 9 Ma and 2044 ± 13 Ma, respectively. The OR rocks are intermediate and felsic, metaluminous, low to moderate alkali. Both rocks are enriched relative to primitive mantle in Rb, Ba, Sr, K, and depleted in Nb and Ti. The OR quartz diorite and IS trondhjemite are characterized by positive values ϵNd(T) = 2.2 and 1.6, respectively. The ORC belongs to intermediate I-type granite with nil or weak crustal contamination, whereas the IS trondhjemite shows adakite-like features that were contaminated with components from earlier magmatic pulses. The low degree of contamination of the hypothetical melt, its basaltic-andesite composition, the elevated potassium and magnesium contents, and trace element patterns of whole rock and zircon, suggest garnet-free lower crust or metasomatized mantle wedge as the magma source for the OR, in a post-collisional setting. The mechanism for the inner stock trondhjemite was probably similar, although the magma source was characterized by the presence of garnet with a moderate degree of melting. [ABSTRACT FROM AUTHOR]