Paleoproterozoic crustal genesis: calc-alkaline magmatism of the Torngat Orogen, Voisey’s Bay area, Labrador

A suite of metaplutonic rocks representing a mafic to intermediate calc-alkaline batholith has been identified in the Voisey’s Bay area of northern Labrador. The suite represents the southern extension of similar metaplutonic rocks occurring along some 500 km of the Paleoproterozoic Torngat Orogen. The rocks have been metamorphosed at upper amphibolite to granulite facies conditions that only mildly disturbed their igneous characteristics. Two magma series are present, one comprising gabbros, diorites and quartz diorites, and another composed of tonalites. The gabbro to quartz diorite series evolved by fractional crystallization of a mantle-derived mafic magma. Two samples of the metaplutonic suite yield U–Pb zircon thermal ionization mass spectrometry (TIMS) ages of 1893±1 and 1890±2 Ma. Based on U–Pb zircon laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS) geochronology, one of these samples contains inherited cores, some 50–100 Ma older than the igneous population, but no Archean cores were found. Metatonalite rocks formed from a separate tonalitic magma during a slightly younger magmatic event dated at 1883±5 Ma. Both the gabbro to quartz diorite and tonalite series rocks are enriched in Rb, Ba, Sr, Zr and light rare earth elements relative to primitive mantle; and have negative Nb–Ta anomalies, which are thought to reflect subduction processes. They have fractionated REE patterns, with the extent of fractionation greatest for the tonalites (chondrite normalized La/Yb=30–50), suggesting that their parent magmas left more residual garnet in their source regions than did the gabbro to quartz diorite magmas. Sr–Nd–Pb isotope compositions of the metaplutonic suite indicate significant involvement of a lower crustal component in their genesis. When combined with previous data for the suite, and excluding data thought to be disturbed by metamorphism, 87Sr/86Sr(1890 Ma)=0.7036–0.7048, εNd(1890 Ma)=+4.7 to −6.9, and μ∗ (time-integrated 238U/204Pb of the mantle and crustal sources) = 7.66–7.98. A model for the metaplutonic suite is presented in which the gabbro to quartz diorite series formed by partial melting of the mantle wedge in a subduction zone and interacted with a lower crustal contaminant, possibly in the source region. The tonalites were derived by partial melting of a mixture of subducting oceanic crust and lower crustal detritus. An important role for lower crustal recycling is thus indicated. Crustal genesis by both mantle wedge and slab melting in the Paleoproterozoic may mark the transition from Archean to post-Archean subduction processes.