An Overview of the Tectonic Evolution of the Indochina block and Granitoid Emplacement, particularly in the central and south Vietnam

Introduction: Vietnam is mainly located within the Indochina block in Southeast Asia. Asmall northern part of Vietnam belongs to the South China block, the southwest part liesadjacent to the Sibumasu block and opens to the East Sea on the east side. Tectonicactivities in Vietnam were very complicated they relate to intense interactions betweenmany geological blocks at different times. Magmatic emplacement is the final and instantproduct of tectonic activities. Methods: Geochemical data analysis from rock samples withinVietnam collected by other researchers has been reused in the scope of this study to verifythe relation between tectonic evolutions and their granitic magmatism. GCD (GeochemicalData Toolkit), an R language program for handling and recalculation of geochemical data. Results: Geochronology and geotectonic model derived from rock analysis have beenascertained main tectonic evolutions of the Indochina. The current granitoidclassification in Vietnam mostly based on petrographical studies. The Nui Cam granitoid isbeing classified as Deo Ca, Dinh Quan granitoid. However, based on trace elements, they aredifferent. They may belong to different granitoid system. Conclusion: Major tectonic eventswithin the Indochina block are well supported by the nature of granitoid emplacements. Petrological studies of these magmatic rocks would bring out valuable information toconfirm and clearly understand the tectonic evolutions of the region. Igneous rocksclassification must based on tectonic fundamental instead of petrographical studies.

Cretaceous reduced granitoids in the Goodpaster Mining District, east central Alaska

The >5 Moz (1 oz (troy) = 31.103 g) gold veins of the Liese Zone and nearby prospects in the Goodpaster Mining District of east central Alaska are spatially and temporally associated with late-Early to Late Cretaceous reduced granitoids that are divided into a granite suite, tonalite suite, and diorite suite in decreasing age. Synkinematic to postkinematic biotite ± hornblende granite, granodiorite, pegmatite, and two-mica granite with accessory garnet compose the granite suite (109–107 Ma). The tonalite suite (107–103 Ma) forms small to large bodies of postkinematic hornblende–biotite granodiorite to tonalite with rare granite. Intruding the granite and parts of the tonalite suites are aplite and pegmatite that grade through sugary pegmatoidal or aplitic quartz veins and finally into quartz veins along strike. The diorite suite (95.4–93.7 Ma) consists of small stocks of diorite to tonalite that intrude the older suites and are inferred to intrude the shallowly dipping auriferous quartz veins. Limited data indicate granitoid emplacement at 5–9 km depths, consistent with formation of the auriferous quartz veins based upon published fluid inclusion data. The weakly peraluminous granite and tonalite suites are distinguished by variable amounts of monazite and zircon whereas the diorite suite is metaluminous, contains <5% magnetite, and lacks monazite. All suites are subalkalic, calc-alkaline and have low magnetic susceptibilities, high large-ion lithophile element/high field-strength element (LILE/HFSE), and depleted Nb and Ti. The granite suite has higher 206Pb/204Pb values (19.4–19.6) than the diorite (19.1). Overall the granite and tonalite suites likely represent melts generated late during crustal thickening that intruded along shallowly dipping faults during exhumation, whereas the diorite suite represents postdeformation melts that underwent less interaction with the old silicic crust.

Evidence for Neoproterozoic orogenesis and early high temperature Scandian deformation events in the southern East Greenland Caledonides

Integrated field structural studies and SHRIMP U–Pb zircon and monazite dating have been undertaken in Renland, west of Scoresby Sund district in the southern part of the East Greenland Caledonides. Southwest Renland is dominated by metasedimentary rocks correlated with the Krummedal supracrustal succession of East Greenland and which on Renland were intruded by augen granites. Krummedal psammite from Renland yielded a spectrum of Mesoproterozoic to Palaeoproterozoic detrital U–Pb zircon dates, the youngest of which indicate deposition of the psammite occurred c. 1000 Ma ago, thus post-dating Grenvillian continent–continent collision in North American Laurentia. These Krummedal metasediments were deformed into regional nappe-scale folds prior to metamorphism, crustal anatexis and genesis of augen granites; an example of the latter has been dated at 915±18 Ma (U–Pb zircon). This demonstrates early Neoproterozoic high-temperature tectono-metamorphism affecting rocks within the southern East Greenland Caledonides, broadly contemporaneous with similar rocks farther north in East Greenland and with Sveconorwegian events on Baltica. Still in southwestern Renland, a later thermal event led to development of uppermost amphibolite to granulite facies metamorphic assemblages, veins and patches of in situ garnetiferous melt-bearing neosome in both metasediments (432±6 Ma, U–Pb zircon) and in the augen granites, and contemporaneous biotite-bearing granite sheets in top-down-to-the-E extensional shear zones (434±5 Ma, U–Pb zircon). Monazites from southwestern Renland record Caledonian thermal events as late as 410−400 Ma. In contrast, southeastern Renland is dominated by quartzofeldspathic migmatites with a strongly Caledonian signature but enclosing relicts of augen granite and retrogressed granulite facies psammitic and pelitic metasediment. There is also a sequence of Caledonian granitoid intrusions. Two samples from a hypersthene monzonite intrusion yielded U–Pb zircon dates of 424±8 Ma and 424±6 Ma. This pluton shows the marginal effects of the regional migmatization and was intruded early in the sequence of granitoid emplacement. An amphibolite facies migmatite, textural evidence from which suggests that it had never hosted granulite facies assemblages, records zircon growth at 423±6 Ma, and closure of monazite by 402±10 Ma. High grade metamorphism, and the protracted sequence of granitoid emplacement and still younger thermal events which together span the period between 430 and 400 Ma may, in part, reflect complicated lithospheric dynamics associated with subduction outboard of the Laurentian margin. Crustal segments carrying the relict evidence of Neoproterozoic and early Caledonian events must then quickly have been thrust northwestwards in foreland-propagating, northwesterly directed thrusts over Cambro-Ordovician platformal sequences on the Laurentian margin. This records the final closure of Iapetus, encroachment of Baltica and continent–continent collision from late Llandovery times (425–430 Ma).

Geology and geochemistry of the Clear Creek intrusion-related gold occurrences, Tintina Gold Province, Yukon, Canada

The Clear Creek gold occurrences lie within deformed lower greenschist-facies rocks of the western Selwyn basin. They consist of auriferous, sheeted quartz veins that cut six Cretaceous stocks and their hornfels. The veins contain 1–2% combined pyrite and arsenopyrite, with lesser pyrrhotite, bismuthinite, and scheelite, as well as 2–5 g/t Au. New 40Ar/39Ar geochronology of hydrothermal micas indicates that the veins formed within 1–2 million years of granitoid emplacement. Fluid inclusion microthermometry defines a parent ore fluid of ~81 mol.% H2O, 14 mol.% CO2, 4 mol.% NaCl ± KCl, and 1 mol.% CH4, which unmixed into a low- and high-salinity immiscible pair. This suggests a more saline parent fluid and a greater degree of fluid unmixing relative to the other occurrences in the eastern Tintina Gold Province. Inclusions trapped in As- and Bi-rich, high-gold grade veins have homogenization temperatures of 300–350°C, whereas inclusions found in more Ag- and Pb-rich veins are characterized by temperatures of 250–300°C. Fluid inclusion geobarometry suggests hydro-fracturing and gold deposition at 5–7 km depth. The δ18O values of quartz samples range from 13–16‰ (per mil) and δ34S for sulfides are also consistent between –3.0‰ and 0‰, with the exception of some outliers from the Contact Zone of the Pukelman stock that indicate a lower temperature second phase of mineralization. It remains uncertain as to whether the Clear Creek ore fluids were exsolved from magmas at depth or from devolatilization reactions within the contact metamorphic aureoles of the intrusions.

Modelling the thermal perturbation of the continental crust after intraplating of thick granitoid sheets: a comparison with the crustal sections in Calabria (Italy)

Thick granitoid sheets represent a considerable percentage of Palaeozoic crustal sections exposed in Calabria. High thermal gradients are recorded in upper and lower crustal regional metamorphic rocks lying at the roof and base of the granitoids. Ages of peak metamorphism and emplacement of granitoids are mostly overlapping, suggesting a connection between magma intrusion and low-pressure metamorphism. To analyse this relationship, thermal perturbation following granitoid emplacement has been modelled. The simulation indicates that, in the upper crust, the thermal perturbation is short-lived. In contrast, in the lower crust temperatures greater than 700°C are maintained for 12 Ma, explaining granulite formation, anatexis and the following nearly isobaric cooling. An even longer perturbation can be achieved introducing the effect of mantle lithosphere thinning into the model.