Derek A. Wyman

Assembly of Archean cratonic mantle lithosphere and crust: plume–arc interaction in the Abitibi–Wawa subduction–accretion complex

Abstract Recent thermodynamic models suggest that direct interaction between mantle plumes and island arcs will enhance long-term arc buoyancy and contribute disproportionately to the crustal record. However, crustal growth models have also proposed a range of differing mechanisms for Archean crust formation that emphasize specific rock types, such as tonalite–trondhjemite–granodiorite (TTG) plutons or high magnesian andesites. Studies in the Abitibi–Wawa subprovince, allow these proposals to be evaluated in the context of the worlds largest greenstone belt. Crustal growth in the southern Superior Province was the product of subduction–accretion tectonics enhanced and modified by mantle plume processes, particularly mantle plume–island arc interaction. High Archean geothermal gradients promoted volumetrically minor slab melts from the earliest phases of the Abitibi–Wawa arc, resulting in the adakite–high magnesium andesite–Niobium–enriched basalt association. However, recent flat subduction models for the formation of adakites also provide important insights into the generation of syn-tectonic Archean TTG batholiths that were probably derived from subducted, rather than accreted, oceanic crust. The distribution of Niobium enriched basalts (NEB) in the southern Superior Province may reflect plume controlled shallow subduction beneath the Abitibi belt that limited melting depths within a restricted asthensopheric mantle tongue. A well-constrained tectonic history, the mantle source requirements of successively-formed components of the Abitibi–Wawa crust, and detailed seismic interpretations of crustal architecture all preclude the presence of an autochthonous mantle lithospheric root beneath the Abitibi–Wawa arc. Instead, the late diapiric ascent of buoyant refractory plume residue and subducted oceanic crust resulted in the coupling of the mantle root and arc crust 10s of million years following batholith emplacement. High-velocity material identified at the base of the crust and centered beneath the Abitibi–Pontiac suture zone is probably Archean aged rather than Proterozoic. It corresponds to minor melts generated during ascent of the plume residue diapir and underplated prior to and during formation of the >200 km thick Abitibi–Wawa continental mantle lithosphere root.

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.

Lode Gold Deposits and Archean Mantle Plume-Island Arc Interaction, Abitibi Subprovince, Canada

In combination with seismic interpretations and geochronological constraints, the association of juvenile arc‐type low‐Ti tholeiitic basalts with komatiites in the southeastern Abitibi subprovince, Canada, supports a history of subduction step back following Late Archean mantle plume–island arc interaction. The resulting paired collision zones preserved abundant komatiites and numerous massive sulphide deposits and established the critical metallogenic features to concentrate the majority of Canadas Precambrian gold resources in a small area of the southern Abitibi subprovince.

Formation of Archean continental lithospheric roots: The role of mantle plumes

Deep lithospheric roots, commonly extending to the diamond stability field, are distinctive characteristics of Archean cratons, but their origin remains controversial. The 2.7 Ga Abitibi greenstone belt initially developed in an intraoceanic setting, and provides unique insights into the formation of Archean continental lithospheric mantle. Data provided by seismic transects, young U-Pb isotopic ages in the exposed deeper crust of the Kapuskasing uplift, and anomalous Ar-Ar isotopic ages from lode gold deposits point to a late coupling of the diamondiferous Abitibi lithospheric mantle to arc crust. These constraints and a lack of Archean crust-forming events following ca. 2650 Ma indicate that the thick Abitibi cratonic root formed by the diapiric ascent of buoyant residue from which mantle plume– derived komatiitic liquids had been extracted. Although substantial variation may have existed in the development of Archean cratonic roots worldwide, the evidence from the Abitibi belt indicates that all such roots were initiated with the coupling of plume-melt residue to greenstone-belt crust.

Pliocene-Quaternary crustal melting in central and northern Tibet and insights into crustal flow

There is considerable controversy over the nature of geophysically recognized low-velocity–high-conductivity zones (LV–HCZs) within the Tibetan crust, and their role in models for the development of the Tibetan Plateau. Here we report petrological and geochemical data on magmas erupted 4.7–0.3 Myr ago in central and northern Tibet, demonstrating that they were generated by partial melting of crustal rocks at temperatures of 700–1,050 °C and pressures of 0.5–1.5 GPa. Thus Pliocene-Quaternary melting of crustal rocks occurred at depths of 15–50 km in areas where the LV–HCZs have been recognized. This provides new petrological evidence that the LV–HCZs are sources of partial melt. It is inferred that crustal melting played a key role in triggering crustal weakening and outward crustal flow in the expansion of the Tibetan Plateau.

Geochemical and isotopic characteristics of Youanmi terrane volcanism: the role of mantle plumes and subduction tectonics in the western Yilgarn Craton

New geochemical and isotopic data for volcanic and shallow-level intrusive rocks of the Murchison Domain in the Youanmi Terrane of the Yilgarn Craton help to clarify some of the stratigraphic ambiguities in the area and are consistent with the most recent lithostratigraphic model. The results reveal the presence of previously unrecognised examples of the boninite-depleted tholeiite suite that, based on εNdT > 4, were likely derived from refractory mantle following plume melting. These new findings support the conclusions of previous studies that magmatism in Archean volcanic successions, including mantle plume-associated and arc sequences, display geochemical evidence of refractory plume-related magma sources throughout their development. The presence of well-preserved boninites in the Polelle Group indicates a significant role for subduction tectonics during the 2825–2700 Ma evolution of the Youanmi Terrane. The subduction zone is tentatively linked to the accretionary processes proposed along the Youanmi Terrane–Narryer Terrane boundary. This model also implies that in the eastern Yilgarn Craton, the initial phases of craton re-assembly may have been driven by this subduction zone prior to the development of an arc off the eastern edge of the craton at ca 2715 Ma.

Late Cretaceous back‐arc extension and arc system evolution in the Gangdese area, southern Tibet: Geochronological, petrological, and Sr‐Nd‐Hf‐O isotopic evidence from Dagze diabases

Back-arc extension and asthenosphere upwelling associated with oceanic lithospheric subduction affect the structure and thermal regime of the arc lithosphere, which often triggers widespread extension-related mafic magmatism. Although it is commonly accepted that the Neo-Tethyan oceanic lithosphere subducted beneath the southern Lhasa block, resulting in the well-known Late Mesozoic Gangdese magmatic arc, the possible role of contemporary back-arc extension and asthenosphere upwelling has been disputed due to a lack of evidence for extension-related mafic magmatism. Here, we report detailed petrological, geochronological, geochemical, and Sr-Nd-Hf-O isotopic data for the Dagze diabases located in the north of the Gangdese district, southern Lhasa block. The zircon U-Pb analyses indicate that they were generated in the Late Cretaceous (ca. 92 Ma) instead of the Eocene (42–38 Ma) as previously believed. These mafic rocks are characterized by variable MgO (4.0–12.2 wt %) and Mg# (42 to 71) values combined with flat to slightly enriched ([La/Yb]N = 1.87–5.23) light rare earth elements (REEs) and relative flat heavy REEs ([Gd/Yb]N = 1.36–1.87) with negative Ta, Nb, and Ti anomalies (e.g., [Nb/La]PM = 0.16–0.51). They also have slightly variable eNd(t) (−1.25 to +4.71) and low initial 87Sr/86Sr (0.7045–0.7058) values with strong positive igneous zircon eHf(t) (+8.0 to +12.1) and low δ18O (5.31–6.12‰) values. The estimated primary melt compositions are similar to peridotite-derived experimental melts. Given their high melting temperature (1332 to 1372°C) and hybrid geochemical characteristics, we propose that the Dagze mafic magmas likely represent mixtures of asthenospheric and enriched lithospheric mantle-derived melts that underwent minor crustal assimilation and fractional crystallization of clinopyroxene. Taking into account the spatial and temporal distribution of Mesozoic mafic-felsic magmatic rocks and regional paleomagnetic and basin data, we suggest that the Dagze mafic rocks resulted from asthenospheric upwelling associated with intracontinental back-arc extension during the rollback of subducted Tethyan oceanic lithosphere in the Late Cretaceous.

Late‐Archean Convergent Margin Volcanism in the Superior Province: A Comparison of the Blake River Group and Confederation Assemblage

The Blake River Group of the Abitibi Subprovince and the Confederation assemblage of the Uchi Subprovince have been characterised as subduction-related volcanic assemblages generated in oceanic and continental margins, respectively. Despite the important differences in their settings, the major element trends of tholeiitic rocks from the two areas resemble each other, and Phanerozoic arcs, more than they do tholeiites from continental rift settings that may be analogues for some Archean greenstone belts. The Blake River Group suite includes magmas generated in relict plume asthenosphere, but the chemical trends also provide evidence of slab melt metasomatism. Primitive rocks in the Confederation assemblage define trace element trends analogous to typical modem arcs, with no indication that melt-mobilized elements such as Zr and Nb have been introduced in significant amounts. Adakite-like rocks were formed as a result of local events such as arc rifting in the South Bay area, or as an indirect consequence of larger events such as global-scale mantle-plume episodes that strongly influenced the southern Abitibi Subprovince and the Blake River Group. Niobium-enriched basalts associated with crustally contaminated rhyolites in the southwest of the South Bay study area are most plausibly linked to rifting of the Uchi Subprovince proto-continent margin. Rhyolites in both areas are fractionation products of mantle-derived melts. In addition to documenting variable crustal contamination, the trace elements systematics of the rhyolites provide evidence of zircon fractionation events that occurred without significant changes in major element compositions. These results are probably attributable to extraction of rhyolitic liquids from crystal mush zones, accompanied by preferential entrainment of zircon crystals, leading to Zr fractionation.

Andesitic crustal growth via mélange partial melting: Evidence from Early Cretaceous arc dioritic/andesitic rocks in southern Qiangtang, central Tibet

Deciphering the petrogenesis of andesitic/dioritic rocks is fundamental to understanding the formation of the continental crust. Here we present detailed petrology, geochronology, major and trace element, Sr–Nd–Hf–O isotope data for the Early Cretaceous (∼122 Ma) dioritic rocks in the Bizha area in southern Qiangtang, Tibet. The dioritic rocks are characterized by large ion lithophile elements, Pb, and light rare earth elements but depletion of high field strength elements with slightly enriched and variable eNd(t) values of −0.01 to −3.31 and initial 87Sr/86Sr isotopic ratios of 0.7053–0.7062. They also have variable magmatic zircon Hf-O isotope compositions (eHf(t) = −5.3 to +3.6 and δ18O = +7.3 to +9.5 ‰). Combined with contemporary andesitic lavas in southern Qiangtang, we suggest that the intermediate magmatic rocks in this area were most probably derived by partial melting of a subduction melange, which is a mixture of mid-oceanic ridge basalts (MORBs), sediments, and mantle wedge peridotites, formed along the interface between the subducted slab and the overlying mantle wedge in a subduction channel before ∼124 Ma. The melange diapir melting was triggered by the asthenospheric upwelling and hot corner flow caused by roll-back of the northward subducted Bangong-Nujiang oceanic slab during the Early Cretaceous. The Early Cretaceous intermediate magmatic rocks in southern Qiangtang have an overall continental crust-like andesitic composition. Therefore, partial melting of melange provides an important support for the generation of andesitic magmas in continental arcs and the “andesite model” for crustal growth.

Zircon U–Pb geochronology and geochemistry of Late Cretaceous–early Eocene granodiorites in the southern Gangdese batholith of Tibet: petrogenesis and implications for geodynamics and Cu ± Au ± Mo mineralization

Cu ± Au ± Mo mineralization is found in multiple intrusive suites in the Gangdese belt of southern Tibet (GBST). However, the petrogenesis of these ore-bearing intrusive rocks remains controversial. Here, we report on mineralization-related Late Cretaceous-early Eocene intrusive rocks in the Chikang–Jirong area, southern Gangdese. Zircon U–Pb analyses indicate that the mainly granodioritic Chikang and Jirong plutons were generated in the Late Cretaceous (ca. 92 Ma) and early Eocene (ca. 53 Ma), respectively. They are high-K calc-alkaline suites with high SiO2 (64.8–68.3 wt.%) and Al2O3 (15.1–15.7 wt.%) contents. Chikang granodiorites are characterized by high Sr (835–957 ppm), Sr/Y (118–140), Mg# (58–60), Cr (21.8–36.6 ppm), and Ni (14.3–22.9 ppm), and low Y (6.0–8.1 ppm), Yb (0.54–0.68 ppm) values with negligible Eu anomalies, which are similar to those of typical slab-derived adakites. The Jirong granodiorites have high SiO2 (64.8–65.3 wt.%) and Na2O + K2O (7.19–7.59 wt.%), and low CaO (2.45–3.69 wt.%) contents, Mg# (47–53) and Sr/Y (14–16) values, along with negative Eu and Ba anomalies. Both Chikang and Jirong granodiorites have similar εHf(t) (7.6–13.1) values. The Chikang granodiorites were most probably produced by partial melting of subducted Neo-Tethyan oceanic crust, and the Jirong granodiorites were possibly generated by partial melting of Gangdese juvenile basaltic crust. In combination with the two peak ages (100–80 and 65–41 Ma) of Gangdese magmatism, we suggest that upwelling asthenosphere, triggered by the rollback and subsequent break-off of subducted Neo-Tethyan oceanic lithosphere, provided the heat for partial melting of subducted slab and arc juvenile crust. Taking into account the contemporaneous occurrence of Gangdese magmatism and Cu ± Au ± Mo mineralization, we conclude that the Late Cretaceous–early Eocene magmatic rocks in the GBST may have a significant potential for Cu ± Au ± Mo mineralization.