A compilation and characterisation of lithics in kimberlite and common maar-diatremes and tephra ring deposits

Maar-diatreme volcanoes are the second-most common type on land, occurring in volcanic fields within all major tectonic environments. Their deposits typically contain an abundance of lithic fragments quarried from the substrate, and many contain large, deep-sourced lithic fragments that were erupted to the surface. Primary volcaniclastic deposits fill the diatreme structure formed during eruption. There is negligible inelastic deformation of diatreme-adjacent country rock, indicating that country rock is removed to create the diatreme structures, either by being shifting downward below observable levels, ejected upward to contribute to surficial deposits, or dissolved and hidden in magma erupted or intruded at depth. No previous study has systematically reviewed and analysed the reported lithic fragments of maar-diatreme systems. We present a comprehensive compilation from published work of lithic characteristics in maar ejecta rings and in diatreme deposits of both common and kimberlite maar-diatremes. For maar-diatremes and their tephra ring deposits, we find no correlations among lithic clast sizes, shapes, depositional sites, and excavation depths. This is difficult to reconcile with models involving systematic diatreme deepening coupled with tephra-ring growth, but consistent with those involving chaotic explosions and mixing. Larger amounts of data are needed to further examine how these types of volcanoes operate.

Zircon Petrochronology of the Kişladaǧ Porphyry Au Deposit (Turkey)

Porphyry deposits typically occur in subduction-related arcs but have more recently also been described in postsubduction, collisional to extensional back-arc settings. These different tectonic environments not only might imply different genetic processes but also seem to result in different metal endowments (e.g., Au rich versus Cu rich). It is therefore relevant, also for exploration purposes, to understand the magmatic processes involved in porphyry formation in these different tectonic environments. This study focuses on the Kişladaǧ porphyry Au (17.4 Moz) deposit in western Anatolia, which is centered on a series of porphyritic monzonite stocks of high-K calc-alkaline to shoshonitic affinity and formed in a continental rifting environment. With 17.4 Moz of Au, Kişladaǧ is of global metallogenic importance and hence a good example for studying the genetic processes associated with porphyry deposits in extensional back-arc settings. We herein combine a comprehensive set of new zircon textural observations, in situ zircon trace element and Hf isotope data, and previously published zircon geochronology to study the magmatic processes associated with porphyry deposit formation at Kişladaǧ. We show that mafic rejuvenation of a slowly crystallizing (between ~15.8 and 14.9 Ma) magma reservoir below Kişladaǧ immediately preceded porphyry deposit formation. Zircon trace elements and geochronology suggest a longer and deeper evolution for the early fertile magmas compared to the later infertile magmas. Magma evolution at Kişladaǧ was accompanied by crustal wall-rock assimilation. Whole-rock Nd and Sr radiogenic isotopes show that increasing asthenosphere-derived melt input under accelerated regional extension caused a loss in fertility of the system over time.

A Compilation and Characterisation of Lithic Populations in Common and Kimberlite Maar-Diatremes and their Tephra Ring Deposits

Maar-diatreme volcanoes are the second-most common type on land, occurring in volcanic fields within all major tectonic environments. Their deposits typically contain an abundance of lithic fragments quarried from the substrate, and many contain large, deep-sourced lithic fragments that were erupted to the surface. Primary volcaniclastic deposits fill the diatreme structure formed during eruption. There is negligible inelastic deformation of diatreme-adjacent country rock, indicating that country rock is removed to create the diatreme structures, either by being shifting downward below observable levels, ejected upward to contribute to surficial deposits, or dissolved and hidden in magma erupted or intruded at depth. No previous study has systematically reviewed and analysed the reported lithic fragments of maar-diatreme systems. We present a comprehensive compilation from published work of lithic characteristics in maar ejecta rims and in diatreme deposits of both common and kimberlite maar-diatremes. For maar-diatremes and their tephra ring deposits, we find no correlations among lithic clast sizes, shapes, depositional sites, and excavation depths. This is difficult to reconcile with models involving systematic diatreme deepening coupled with tephra-ring growth, but consistent with those involving chaotic explosions and mixing. Larger amounts of data are needed to further examine how these types of volcanoes operate.

Geochemistry of Granites from Chail Group of Garhwal Region, Lesser Himalaya, NW India

Paleoproterozoic granites are well exposed in the Chail group of Garhwal region, Lesser Himalaya crystalline sequences (LCHS). These granites are less studied in terms of geochemical classification and tectonic settings. In the present work, we carried out the geochemical analysis of granites of the Chail group from the Chirbatiya-Khal and Ghuttu areas. All the samples have high SiO2 (73.24 ̶79.1 wt %), Al2O3 (11.2 ̶12.95 wt %), K2O (3.8 ̶5.9 wt %) and low P2O5 (0.11 ̶0.24 wt %), CaO (0.21 ̶1.02 wt %), and Na2O (2.2-3.03 wt %; exceptionally low in one sample, that is 0.009 wt %) contents. The A/CNK values for the samples are range from 1.19 to 2.91, characteristic of S-type granites. REE patterns for these granites are moderately fractionated with an average (La/Yb)N ̴8.21 and europium anomaly (Eu/Eu*) ̴0.15. The tectonic settings of the studied granite suggest that they are formed in syn-collision tectonic environments.