Structure and performance of PM ultrahard high speed steel: A.L.Wojcieszynski, W.B.Eisen. (Crucible Research, USA.)

Abstract

Four distinct granitic magma series developed systematically in space and time during the evolution of the Southern Volcanic Zone of the Archean Abitibi granite-greenstone belt, Canada (∼ 2.75–2.67 Ga).Minor synvolcanic tonalite-trondhjemite-granodiorite (TTG) intrusions, such as the Lac Flavrian and Bourlamaque plutons, were emplaced at ∼ 2700 Ma at pressures of ∼ 1 kbar. Synvolcanic TTG's are chemically similar to Barker's low-Al type Archean TTG, and are characterised by extremely low K2O (< 1.5 wt%), Rb (< 50 ppm) and Sr (< 200 ppm) concentrations, enhanced TiO2 (0.5 wt%), Fe2O3 (4.4 wt%), Nb, Y and Sc, flat REE patterns, and negative Eu anomalies that increase with HREE abundance. The TTG's are interpreted to have originated by fractionation of basaltic magma from partial melting of a depleted mantle source in a supra-subduction zone environment.Voluminous tonalite-granodiorite-granite and quartz monzonite (TGGM series), exemplified by the Round Lake and Lac Abitibi batholiths, developed syntectonically at ∼ 2690 Ma. Compositionally, the TGGM-series granites resemble Archean and Phanerozoic high-Al type TTG; they feature low K2O (1–3 wt%), Rb (10–50 ppm), Ba (< 1000 ppm) and U contents, enhanced Sr, low Ba/Sr (0.5–1.25) ratios, normalised depletion of Ta, Nb and Ti, and strongly fractionated REE patterns [(LaYb)n = 49-21] with minor positive or no Eu anomalies. Syntectonic TGGM's formed by partial melting of an amphibolite and/or quartz eclogite basaltic protolith from a subducting or subcreted slab.Late-tectonic quartz syenite-quartz monzonite-granite (SMG) series range from batholithic dimensions (e.g., Watebeag) to stocks (e.g., Garrison), and were emplaced at shallow levels (⩽ 1 kbar) between 2681 to 2676 Ma. In composition they are similar to the TGGM series, including REE distributions, but feature lower CaO(K2O + Na2O) and greater concentrations of Rb, Ba, Th and U. This granitic series may represent mixtures of magmas derived from a “metasomatised” mantle and TGGM-type sources.Late- to post-tectonic alkali feldspar syenite-alkali feldspar quartz syenite (SS) series (2680-2670 Ma) are one expression of shoshonitic magmatism most prevalent along regional translithospheric strike-slip structures. Comagmatic trachytes unconformably overlie the deformed and metamorphosed greenstone belt. The most primitive magmas (SiO2 ⩽ 65 wt%) are characterised by the conjunction of extreme large-ion lithophile element (LILE) and light REE (LREE) enrichment, strongly fractionated REE patterns, and relative depletions of Ta, Nb and Ti, with enhanced Cr, Co and Ni abundances; evolved phases (SiO2 ⩾ 65 wt%) possess lower contents of compatible and incompatible elements stemming from differentiation. Primitive magmas of the SS series may have been derived from low degrees of partial melting of a depleted mantle wedge, previously metasomatised by alkali-rich fluids released from a subducting slab.In the adjacent Pontiac subprovince, two granitic series predominate: a syn- to late-tectonic monzodiorite-monzonite-granodiorite-syenite (MMGS) series (2690-2670 Ma), and a garnet-muscovite granite (GMG) series (∼ 2645 Ma). The MMGS's have comparable major-element, LILE and REE contents to Abitibi SMG and evolved SS granitic series, but are distinguished by strong correlations of U, Th, Zr and Hf with SiO2, extremely high Ba/Th ratios, and by co-enrichment of Cr, Co and Ni with LREE and Li and Cs in some MMGS phases. GMG series, which are associated with migmatites and Mo,U,Be,Li-pegmatites are characterised by K2ONa2O ⩾ 1, restricted SiO2 range (69–75 wt%), strong enrichments of Rb, Li, Cs, Ta, Nb, Th and U, and moderately fractionated REE's [(LaYb)n = 0.9–16], with pronounced negative Eu anomalies: they are similar to collisional S-type granites and represent infracrustal melting of mature metasediments during a continent-arc collision involving thrusting of the Pontiac subprovince beneath the Abitibi Southern Volcanic Zone.Collectively, the six granitic series reflect a transition from extensional supra-subduction zone magmatism with depleted mantle sources, to volcanic arc magmatism involving subducting slab during early collision, to alkaline granites derived from metasomatised mantle sources beneath a mature arc undergoing extension, and finally to magmatism resulting from terrane-terrane collision over a period of ∼ 70 Ma.