Post-subduction Setting Research Articles

Porphyry and Related Deposits in Subduction and Post‐Subduction Settings

Porphyry and Related Deposits in Subduction and Post‐Subduction Settings

Tectonic, magmatic, and metallogenic evolution of the Tethyan orogen: From subduction to collision

This paper reviews the tectonic, magmatic, and metallogenic history of the Tethyan orogen from the Carpathians to Indochina. Focus is placed on the formation of porphyry Cu±Mo±Au deposits, as being the most characteristic mineral deposit type formed during both subduction and collisional processes in this region. Relatively little is known about the history of the Paleotethys ocean, which opened and closed between Gondwana and Eurasia in the Paleozoic, and few ore deposits are preserved from this period. The Neotethyan ocean opened in the Permian–Early Triassic as the Cimmerian continental fragments (the cores of Turkey, Iran, Tibet, and Indochina) rifted from the northern Gondwana margin and drifted northwards. These microcontinents docked with the Eurasian margin at various points in the Mesozoic and Cenozoic, and formed a complex archipelago involving several small back-arc basins and remnants of the Paleotethyan ocean. The main Neotethyan ocean and these smaller basins were largely eliminated by collision with India and Africa–Arabia in the early Eocene and early-mid Miocene, respectively, although Neotethyan subduction continues beneath the Hellenic arc and the Makran.The majority of porphyry-type deposits are found in association with Neotethyan subduction (mainly in the Mesozoic and Paleogene), and syn- to post-collisional events in the mid-Paleogene to Neogene. They are found throughout the orogen, but some sections are particularly well-endowed, including the Carpathians–Balkans–Rhodopes, eastern Turkey–Lesser Caucasus–NW Iran, SE Iran–SW Pakistan, southern Tibet, and SE Tibet–Indochina. Other sections that appear barren may reflect deeper levels of erosion, young sedimentary cover, or lack of exploration, although there may also be real reasons for low prospectivity in some areas, such as minimal subduction (e.g., the western Mediterranean region) or lithospheric underthrusting (as proposed in western Tibet).Over the last decade, improved geochronological constraints on the timing of ore formation and key tectonic events have revealed that many porphyry deposits that were previously assumed to be subduction-related are in fact broadly collision-related, some forming in back-arc settings in advance of collision, some during collision, and others during post-collisional processes such as orogenic collapse and/or delamination of subcontinental mantle lithosphere. While the formation of subduction-related porphyries is quite well understood, collisional metallogeny is more complex, and may involve a number of different processes or sources. These include melting of: orogenically thickened crust; previously subduction-modified lithosphere (including metasomatized mantle, underplated mafic rocks, or lower crustal arc plutons and cumulates); or upwelling asthenosphere (e.g., in response to delamination, slab breakoff, back-arc extension, or orogenic collapse).The most fertile sources for syn- and post-collisional porphyry deposits appear to be subduction-modified lithosphere, because these hydrated lithologies melt at relatively low temperatures during later tectonomagmatic events, and retain the oxidized and relatively metalliferous character of the original arc magmatism. Unusually metallically enriched lithospheric sources do not seem to be required, but the amount of residual sulfide phases in these rocks may control metal ratios (e.g., Cu:Au) in subsequent magmatic hydrothermal ore deposits. Relatively Au-rich deposits potentially form in these settings, as observed in the Carpathians (e.g., Roşia Montană), Turkey (Kisladag, Çöpler), and Iran (Sari Gunay, Dalli), although the majority of syn- and post-collisional porphyries are Cu–Mo-rich, and resemble normal subduction-related deposits (e.g., in the Gangdese belt of southern Tibet). This similarity extends to the associated igneous rocks, which, being derived from subduction-modified sources, largely retain the geochemical and isotopic character of those original arc magmas. While still retaining a broadly calc-alkaline character, these rocks may extend to mildly alkaline (shoshonitic) compositions, and may display adakite-like trace element signatures (high Sr/Y and La/Yb ratios) reflecting melting of deep crustal garnet amphibolitic sources. But they are otherwise hard to distinguish from normal subduction-related magmas.Small, post-collisional mafic, alkaline volcanic centers are common throughout the orogen, but for the most part appear to be barren. However, similar rocks in other post-subduction settings around the world are associated with important alkalic-type porphyry and epithermal Au±Cu deposits, and the potential for discovery of such deposits in the Tethyan orogen should not be overlooked.

A modified genetic model for the Huangshandong magmatic sulfide deposit in the Central Asian Orogenic Belt, Xinjiang, western China

The Huangshandong Ni–Cu deposit is the largest magmatic sulfide deposit discovered to date in the Central Asian Orogenic Belt in northern Xinjiang, western China. The host intrusion is a 274-Ma composite mafic–ultramafic intrusion consisting of four separate intrusive units: a large layered gabbroic sequence (phase I), a sheet-like ultramafic body (phase II), a dyke-like gabbronorite body (phase III), and an irregular ultramafic unit (phase IV). Important sulfide mineralization is present in the last three intrusive units, predominantly as disseminated and net-textured sulfides (pyrrhotite, pentlandite, and chalcopyrite). The Huangshandong mafic–ultramafic intrusive rocks are characterized by arc-like geochemical signatures such as low Ca content in olivine and negative Nb–Ta anomalies in whole rocks. This, together with a post-subduction setting for the East Tianshan in the Permian, suggests that the source mantle was modified previously by slab-derived fluids in the Carboniferous. The mantle-derived magma was ponded in a staging chamber in the lower part of the newly formed arc crust. The first batch of magma to arrive at Huangshandong was most fractionated and depleted in Ni, crystallizing Fe-rich and Ni-depleted olivine (Fo67, <300 ppm Ni). The second batch of magma was more primitive, crystallizing more primitive olivine (Fo81–84). The third batch of magma was also highly fractionated and depleted in Ni, crystallizing Fe-rich and Ni-depleted olivine (Fo72, ~600 ppm Ni). The final batch of magma became more primitive again, crystallizing the most primitive olivine (Fo81–86). The occurrence of rounded sulfide inclusions in olivine primocrysts in the Huangshandong ultramafic rocks indicates that immiscible sulfide liquid droplets were present during olivine crystallization. The Ni tenors of disseminated sulfide ores in the gabbronorite dyke vary mainly between 5 and 8 wt%, which are too high to have been produced by the parental magma of the dyke. The Ni, Cu, and platinum-group elements (PGE) tenors of disseminated sulfide ores in the dyke (phase III) and the ultramafic sheet (phase II) are remarkably similar. These observations, together with the sequence of magma emplacement, suggest that the sulfide liquids entrapped in the magma of the dyke formed at depth by a previous pulse of more primitive magma. The estimated parental magma for the most primitive lherzolites in the Huangshandong intrusion contains 10 wt% MgO. Modeling shows that sulfide saturation in the parental magma of the Huangshandong lherzolites could have resulted from fractional crystallization. Significant PGE depletions relative to Ni and Cu in the disseminated sulfide ores of the Huangshandong deposit may be due to sulfide retention in the source mantle.

Sublithospheric small-scale convection—A mechanism for collision zone magmatism

We studied the effect of increased water content on the dynamics of the lithosphere-asthenosphere boundary in a postsubduction setting. Results from numerical mantle convection models show that the resultant decrease in mantle viscosity and the peridotite solidus produce small-scale convection at the lithosphere-asthenosphere boundary and magmatism that follows the spatially and temporally scattered style and volumes typical for collision magmatism, such as the late Cenozoic volcanism of the Turkish-Iranian Plateau. An inherent feature in small-scale convection is its chaotic nature that can lead to temporally isolated volcanic centers tens of millions of years after initial continental collision, without evident tectonic cause. We also conclude that water input into the upper mantle during and after subduction under the circum-Mediterranean area and the Tibetan Plateau can account for the observed magmatism in these areas. Only fractions (200–600 ppm) of the water input need to be retained after subduction to induce small-scale convection and magmatism on the scale of those observed from the Turkish-Iranian Plateau.

Partial Melting of Thickened or Delaminated Lower Crust in the Middle of Eastern China: Implications for Cu‐Au Mineralization

Field relations, isotope systematics, and plate tectonic reconstructions require that felsic adakites in the Yangtze Block and the Dabie Orogen, eastern China, were not derived from a subducting slab, despite the signature of a mantle component in the contemporaneous mafic adakite hosts of Cu‐Au deposits in the same areas. The apparently contradictory requirements are accounted for by (a) a deep crustal melting origin for barren adakites and (b) a crustal delamination origin, followed by ascent through lithospheric mantle, for adakites associated with mineralization. The crustal delamination process associated with the prospective porphyries duplicates the metallogenically essential aspects of the subduction environment. The importance of adakitic magmas in the genesis of porphyry‐style Cu‐Au deposits is affirmed by these findings, but the range of prospective tectonic environments is extended to include an important intraplate, postsubduction setting. The porphyries of eastern China demonstrate a previously unrecognized relationship between one particular tectonic environment and porphyry‐style mineralization that may occur elsewhere.

Late Archaean granites: a typology based on the Dharwar Craton (India)

Abstract Extensive field work in the Eastern Dharwar Craton, associated with petrographic and geochemical (major and trace elements) investigations, allows four main types of Late Archaean granitoids to be distinguished. (1) Na-rich granitoids of trondhjemitic, tonalitic and granodioritic composition (TTG) that are characterised by strongly fractionated REE patterns and low HREE contents and generally interpreted as “slab melts” generated by partial melting of metamorphosed hydrated basalt, most likely in a subduction environment. (2) Sanukitoids, which are K- and Mg-rich monzonites and granodiorites with TTG-like REE patterns associated with marked LILE-enrichment, and considered to result from the reaction of slab melts generated in a subduction environment with, and assimilation of, mantle wedge peridotite. (3) Uncommon high-HFSE, Mg and K granites with strongly REE and LILE-enrichment that, probably formed by partial melting of an enriched mantle source; unlike in the genesis of sanukitoids, in this case the slab melt is considered to be wholly consumed by reaction with mantle minerals, resulting in mantle-enrichment. Subsequent melting of this enriched mantle (probably in a post-subduction setting) gives rise to high-HFSE, Mg and K magmas. As demonstrated for the Closepet Granite (Dharwar Craton), the hot mantle-derived magma can induce melting of continental crust and then mix with the anatectic products. (4) K-rich, Mg-poor anatectic biotite-granites with REE patterns that are less fractionated and show negative Eu anomalies. These granites result from the remelting of old basement or recently accreted plutons, both with TTG compositions. Such anatexis can occur either in a subduction or in a post-subduction environment. This typology, based on the well-exposed Dharwar Craton, can be extended to Late Archaean granitoids from all over the world. About such 500 analysis were compiled, and a number of discriminant diagrams, based on both major and trace elements, are proposed. The most likely tectonic setting for the observed rock types is an accretionary orogen with accretion of continental blocks above a subduction system, followed by thermal reworking of the newly accreted continental material. However, it has been found that Archaean “subduction-related” granitoids are significantly different from their modern counterparts, implying progressive changes in the modes of magma generation at convergent margins from the Archaean to the present.