Short‐duration contact metamorphism of calcareous sedimentary rocks by Neoproterozoic Franklin gabbro sills and dykes on Victoria Island, Canada

Abstract

This contribution addresses contact metamorphism and fluid flow in calcareous rocks of the Neoproterozoic Shaler Supergroup on Victoria Island, Arctic Canada. These processes occurred due to intrusion of gabbroic sills and dykes at c. 720 Ma during the Franklin magmatic event, which was associated with the break‐up of Rodinia. The intrusive sheets (sills and dykes) are a few metres to ∼50 m thick. Metasedimentary rocks were examined in three locations with very good exposures of vertical dykes feeding horizontal sills, the Northern Feeder Dyke (NFD) complex, the Southern Feeder Dyke (SFD) complex and the Uhuk Massif. In the NFD and SFD complexes, protoliths were limestones and dolostones with minor silicates, and at the Uhuk Massif, the protoliths were silty dolostones. At the time of magma emplacement, these locations were at depths of 1–4 km. The widths of contact aureoles are only several decametres wide, commensurate with thicknesses of the dykes and sills. Splays of tremolite mark incipient metamorphism. Highest grade rocks in the NFD and SFD complexes contain the prograde assemblage diopside + phlogopite whereas at Uhuk they contain the assemblage vesuvianite + garnet + diopside. The assemblages are successfully modelled with T–X(CO2)fluid pseudosections that suggest achievement of CO2‐rich fluid compositions due to early decarbonation reactions, followed by influx of aqueous fluids after peak metamorphism. Rapid heating of host rocks and short near‐peak temperature intervals are demonstrated by the prevalent morphology of diopside as radial splays of acicular crystals that appear to pseudomorph tremolite and by incomplete recrystallization of calcite in marbles. Calcsilicates in the roof of one sill at Uhuk experienced metasomatic influx of Fe that is evidenced by nearly pure andradite rims on grossular garnet. Vesuvianite, which overgrew the grossular portions of garnet, also contains ferric iron. Vesuvianite was partially consumed during retrograde growth of serpentine and andradite. The occurrence of serpentine in high‐grade portions of aureoles is consistent with eventual levelling‐off of temperatures between 350 and 400 °C, an inference that is supported by modelled conductive heat transfer from the cooling magma sheets. Focused fluid flow near intrusion‐wall rock contacts is demonstrated by narrow zones of anomalously low δ13C and δ18O values of carbonate minerals. Although the up to 5‰ decrease of both δ13C and δ18O values from sedimentary values is much smaller than is typical for calcsilicate aureoles around large plutons, it is greater than what could have been achieved by decarbonation alone. The decrease in δ13C is attributed to fluid‐mediated exchange with organic low‐13C carbon that is dispersed through the unmetamorphosed rocks and the decrease in δ18O is attributed to fluid‐mediated isotopic exchange with the gabbroic intrusive sheets. This study shows that when gabbroic sills and dykes intrude a sedimentary basin, (i) contact aureoles are likely to be narrow, only on the scale of ...

Sulfide Immiscibility Induced by Wall-Rock Assimilation in a Fault-Guided Basaltic Feeder System, Franklin Large Igneous Province, Victoria Island (Arctic Canada)

“…The calc-silicates in the skarn suggest intense contact metamorphism during sill emplacement . The Fe oxide facies of the skarn was inferred to represent the expulsion of late Fe-rich magmatic fluids from the sills into the roof panel limestones at the Uhuk Massif (Bédard et al, 2012;Nabelek et al, 2013). If the analogy with the field relationships at the Uhuk Massif holds, then the skarn facies at Zone D would be remnants of a metamorphic aureole that sheathed the conduit system that fed the Zone D sill.…”

The economic potential of some metacarbonate rocks in Akoko-Edo, Edo state Nigeria

“…Carbonatites, and dolomite or marble are the most commonly occurring metacarbonate rocks. The carbonatites are strongly identified with igneous activity; dolomite and limestone are sedimentary (Hsu 1966); and marble is the transformative result of carbonaceous sediments subjected to contact or dynamic metamorphism (Nabelek et al 2013). Hydrochloric acid (HCl) test is a standard test used for the identification of carbonate mineral; it gives a strong reaction to marble minerals (Robert 1979).…”

The geochemical effects of olivine slurry replenishment and dolostone assimilation in the plumbing system of the Franklin Large Igneous Province, Victoria Island, Arctic Canada

“…Pb-isotope compositions suggest that relative to the average Type-1 magma, this final pulse assimilated small amounts (<10 %) of dolostone (Beard et al submitted). The Franklin intrusions of the Minto Inlier are hosted by a predominantly carbonate/dolostone sedimentary sequence (Thomson et al 2014), and calc-silicate reaction products are locally prominent (Nabelek et al 2013). Bédard et al (2012) report field and thin-section evidence of dolostone assimilation in the UM.…”

“…The Franklin LIP has a sill-dominated magmatic plumbing system with local fault-mediated shifts in emplacement level that facilitated magma transgression (Bédard et al 2012). Franklin sills were emplaced primarily into limestones and dolostones, and calc-silicate skarns that formed by magma-carbonate interaction are locally prominent (Nabelek et al 2013). Hayes et al (2015) documented the development of firstorder modal layering in a composite Franklin sill as a result of two magmatic pulses.…”

“…Both have olivine-enriched bases and were emplaced at the same stratigraphic level, just above the Fort Collinson Formation marker unit. The WUS is adjacent to a prominent fault-guided magma upflow zone known as the Uhuk Massif (UM: described in Bédard et al (2012)), which preserves calc-silicate reaction facies at some intrusive contacts (Nabelek et al 2013). These three olivine-enriched sills (WUS, PS, and LPS) are collectively termed the Fort Collinson Sill Complex (FCSC).…”