An Archean arc basalt–Nb-enriched basalt–adakite association: the 2.7 Ga Confederation assemblage of the Birch–Uchi greenstone belt, Superior Province

Abstract

The Uchi subprovince of the Archean Superior Province is a series of greenstone belts extending 600 km east±west along the southern margin of the North Caribou Terrane protocontinent. The 2.7 Ga Confederation tectonostratigraphic assemblage of the Birch±Uchi greenstone belt, northwest Ontario, is dominated by volcanic suites of ma®c, intermediate and felsic composition. Tholeiitic basalts range compositionally from Mg# 59±26 evolving continuously to greater REE contents (La 2±19 ppm; Th/La pm~1 ), with small negative Nb anomalies. Primitive tholeiites are similar to modern intraoceanic arc basalts, whereas evolved members extend to greater concentrations of Ti, Zr, V, Sc, and Y, and lower Ti/Zr, but higher Ti/Sc and Ti/V ratios characteristic of back arc basalts. Calc-alkaline basalts to dacites are characterised by more fractionated REE (La/Yb n 1±8), high Th/Nb pm ratios and deeper negative Nb anomalies; they plot with modern oceanic arc basalts and some may qualify as high magnesium andesites. The two suites are interpreted as a paired arc±back arc sequence. A third group of Nb-enriched basalts (NEB; Nb 9±18 ppm) extend to extremely high TiO 2 , Ta, P 2 O 5 , Sc and V contents, with strongly fractionated REE and ratios of Nb/Ta and Zr/Hf greater than primitive mantle values whereas Zr/ Sm ratios are lower. The most abundant rhyolitic suite has extremely enriched but¯at trace element patterns and is interpreted as strongly fractionated tholeiitic basalt liquids. A second group are compositionally similar to Cenozoic adakites and Archean high-Al, high-La/Yb n tonalites; they possess Yb £ 0.4 ppm, Y £ 6 ppm and Sc £ 8 ppm, with La/Yb n of 19±30 and Zr/Sm of 50±59. They are interpreted as melts of ocean lithosphere basaltic crust in a hot shallow subduction zone. Adakites are associated with NEB in Cenozoic arcs where there is shallow subduction of young and/or hot ocean lithosphere, often with oblique subduction. Slab melt adakites erupt, or metasomatise sub-arc mantle peridotite to generate an HFSE-enriched source that subsequently melts during induced mantle convection. The Archean adakite±NEB association erupted during development of the tholeiitic to calc-alkaline arc and its associated back arc. Their coexistence in the Confederation assemblage of the Birch±Uchi greenstone belt implies convergent margin processes similar to those in Cenozoic arcs.

Neoarchean siliceous high-Mg basalt (SHMB) from the Taishan granite–greenstone terrane, Eastern North China Craton: Petrogenesis and tectonic implications

“…and/or boninites in Archean terrains, could provide an analogy to modern-style mechanisms and further constraints on the origin of TTGs and their geodynamic settings. Indeed, remarkably similar magmatic associations involving calc-alkaline lavas, adakites, and/or Nb-enriched basalts, boninites or high-Mg andesites have been widely described from Archean greenstone belts worldwide, including in the Superior (Hollings and Kerrich, 2000;Polat and Kerrich, 2001;Ujike and Goodwin, 2003;Ujike et al, 2007), Slave, Baltic, Dharwar Naqvi et al, 2006;Manikyamba and Khanna, 2007), Tanzania (Manya et al, 2007) and North China cratons (Wang et al, 2009;Peng et al, 2012a, and this study), as summarized in Table 4. For example, island arc tholeiite (IAT) and boninite-like rocks in suprasubduction zone (SSZ) ophiolites have been identified from the ∼3.8 Ga Isua supracrustal belt in Greenland Furnes et al, 2009), suggesting that Phanerozoic-like subduction processes were likely operating as early as 3.7-3.8 Ga. Also, the recent recognition in the Whundo Group in the Pilbara Craton of Western Australia (Smithies et al, 2007) of a ∼3.12 Ga association including subduction slab-derived adakites and sub-arc mantle-derived boninites, with Nb-enriched basalts and high-Mg andesites, supports the view that modernstyle subduction took place in some Archean terranes at least before 3.0 Ga, rather than after 3.0 Ga as initially suggested by Smithies (2000).…”

“…However, increasing evidence has been documented that the volumetrically more minor, metamorphosed volcanic components associated with TTGs could provide significant information on their origin in Archean granite-greenstone belts (e.g., Kerrich et al, 1998;Hollings and Kerrich, 2000;Polat and Kerrich, 2001;Polat et al, 2002;Manikyamba et al, , 2009Manikyamba and Khanna, 2007). Notably, the identification of similar rock assemblages in Archean greenstone belts, which might be analogous to modern successions, has played a critical role in constraining their petrogenetic mechanism and thus of continental growth in the Archean.…”

The Paleozoic tectonic evolution and metallogenesis of the northern margin of East Junggar, Central Asia Orogenic Belt: Geochronological and geochemical constraints from igneous rocks of the Qiaoxiahala Fe-Cu deposit

“…The relative enrichments of LILEs and LREEs over HFSEs and HREEs, and the high Ba (960-1160 ppm) and Ba/Th (450-773, >170) suggest a slab-derived fluid influence (Plank and Langmuir, 1998). The high Ce/Th (13.67-16.67, >8), low Th/Yb (0.81-1.26, <2) and Th/La (0.14-0.16, <0.2) and absence of negative Ce anomaly (0.98-1.02, mean 1.00) and negative Nb anomaly suggest insignificant contribution from subducted oceanic crust and sediments (Plank and Langmuir, 1998;Hollings and Kerrich, 2000). In summary, the Qiaoxiahala diorite porphyry was likely the fractional crystallization product of parental basaltic magmas, with the addition of lower crustal materials and pre-exist sedimentary rocks with Precambrian zircons.…”

“…Elevated Ce/Th (17.3-46.6) and Ba/Th ratios (123-2400), and the absence of pronounced negative Ce anomalies (0.88-1.10, mean 0.95) suggest the absence of significant subducted sediment input, as sources modified by subducted sediments are typically characterized by elevated Th, low Ce/Th ($8) and Ba/Th ($111), as well as negative Ce anomalies (Plank and Langmuir, 1998). The depleted Nb (1.94-5.80 ppm) contents in the basaltic rocks suggest the absence of Nb-enriched basalts in the Qiaoxiahala deposit, suggesting that melts derived from subducted oceanic crust have not been erupted in the arc (Hollings and Kerrich, 2000;Wyman et al, 2000). Moreover, the enrichment of LILEs and LREEs over HFSEs and HREEs indicate subduction-related metasomatism by the LILEs-and LREEs-bearing slab fluids (Schiano et al, 1995;Münker et al, 2004).…”

Petrogenesis of gold-mineralized magmatic rocks of the Taerbieke area, northwestern Tianshan (western China): Constraints from geochronology, geochemistry and Sr–Nd–Pb–Hf isotopic compositions

“…9a-c. HMA types commonly include boninites (Crawford et al, 1989), bajaites (Rogers et al, 1985;Rogers and Saunders, 1989), high-Mg adakites (Defant and Drummond, 1990;Yogodzinski et al, 1994Yogodzinski et al, , 1995Yogodzinski and Kelemen, 1998;Hollings and Kerrich, 2000;Martin et al, 2005;Wang et al, 2006aWang et al, , 2006bWang et al, , 2011bZhang et al, 2010a;Xu et al, 2012) and sanukitoids (Shirey and Hanson, 1984). The Taerbieke and Axi andesites are distinct from boninites in that they show higher TiO 2 (>0.48 wt.%) and Al 2 O 3 (15.39 wt.%) contents (Table 2) (Crawford et al, 1989;Taylor et al, 1994).…”