Lithium isotopic and fluid mobile trace element systematics of the Bay of Islands altered forearc upper to lower ophiolitic crust

Abstract

Bulk rock Li isotope and select fluid mobile element traverses through the crustal section of the Late Cambrian Bay of Islands Ophiolite Complex (BOIC) in the Taconic Humber Arm Allochthon in the Newfoundland Appalachians are presented to examine the extent and conditions of alteration at different pseudo-stratigraphic levels in the ancient forearc oceanic crust that was obducted during the Middle Ordovician.N-MORB normalized enrichment factors (EFs) calculated for Li and fluid mobile elements (FMEs) show variable extents of enrichments or depletions from shallow to deep oceanic crustal sections in the BOIC that are consistent with a broad range of alteration-related δ7Li values from −2.97 to +20.69 ‰ (vs. unaltered MORB range of +1.5 to +6.1 ‰ with a mean of +3.4 ± 1.4 ‰). EFs and fluid-rock reaction modeling for Li contents and δ7Li values indicate alteration dominantly controlled enrichments and depletions of Li isotopic and FMEs systematics through all crustal levels of the BOIC ophiolite. Alteration fluids are dominated by modern seawater-like δ7Li composition rather than the recently proposed Cambrian seawater that is based on ancient carbonate shell compositions.BOIC basaltic samples proximal to the York Harbor Mine (YHM), similar to the classic mafic Cu-Zn Cyprus-type volcanic massive sulfide (VMS) deposits, show light δ7Li values of −2.97 to +1.59 ‰, but highly enriched Li contents (14.0–40.7 μg/g). Modeling results indicate that these YHM basalts were altered by metal-rich magmatic fluids, in part, derived from deeper plagiogranite intrusions near the base of the sheeted dikes that are possibly admixed with seawater-derived hydrothermal fluids below the VMS deposit. Magmatic volatile exsolution may be associated with the crystallization of directly underlying subvolcanic silicic intrusions, resulting in over-pressuring and fracturing of the carapace that may assist in localizing the high temperature (400–200 °C) upflow zones of the seafloor hydrothermal system.The BOIC layered gabbroic rocks also show evidence of variable extent of alteration as reflected by both Li content (EF both >1 and < 1) and δ7Li composition (+0.79 to +9.47 ‰), likely indicating high-temperature amphibolite (800–600 °C within the ductile deformation field) to lower temperature greenschist facies paragenesis, with down-temperature complex retrograde petrogenetic lineages. Mylonites and ductile shear zones commonly show heavier δ7Li values between +5.47 and + 18.83 ‰. We suggest that mylonite zones and coarser-grained high-strain zones allowed near-axis fluid penetration at high to lower temperatures that extended to the basal gabbroic sections in the crust during ongoing igneous activity. Shear zones were associated with rheologic weakening and initial permeable pathways for hydrothermal fluids. Shallow on-axis and deep off-axis hydrothermal circulation likely occurred at a relatively fast-spreading rate, which may account for the significant shallow to deep crustal heat extraction as predicted by the multiple sill model of lithospheric cooling.