Geochemistry of subduction zone serpentinites: A review

Orogenic, ophiolitic, and abyssal peridotites represent subcontinental, suboceanic, and subarc mantle rocks that were exhumed to the surface in various tectonic settings. These rocks provide coverage of vast regions of the Earth's upper mantle that are sparsely sampled by mantle xenoliths. They notably allow the observation of a wide range of lithospheric mantle compositions, including cratonic roots and subduction mantle wedges (high-pressure orogenic garnet lherzolites), variably rejuvenated subcontinental lithosphere (lower-pressure orogenic spinel and plagioclase lherzolites), and newly accreted oceanic lithosphere (ophiolitic mantle and abyssal peridotites). It is shown here that most of geochemical variability recorded by these mantle rocks is attributable to melt processes associated with partial melting and asthenosphere–lithosphere interactions. Rather than remnants of pristine mantle, the fertile orogenic lherzolites are now widely considered as former refractory lithospheric mantle refertilized by upwelling partial melt.

Orogenic, ophiolitic and abyssal peridotites

Rock-forming serpentine minerals form flat, cylindrical, and corrugated crystal microstructures, which reflect energetically efficient layering of alternate tetrahedral and octahedral sheets. Serpentinization of peridotite involves internal buffering of the pore fluid, reduction of oxygen fugacity, and partial oxidation of Fe 2+ to Fe 3+ . Sluggish MgFe diffusion in olivine causes precipitation of magnetite and release of H 2 . The tectonic environment of the serpentinization process dictates the abundance of fluid-mobile elements in serpentinites. Similar enrichment patterns of fluid-mobile elements in mantle-wedge serpentinites and arc magmas suggest a linkage between the dehydration of serpentinite and arc magmatism.

Serpentinite: What, Why, Where?

The mineral system approach applied to magmatic Ni-Cu-PGE sulphide deposits

Tectonic significance of serpentinites

Serpentinites in a Tertiary subduction complex in the northern Dominican Republic contain low concentrations of incompatible elements in bulk-rock compositions and high Mg in relict silicate minerals. The forsterite component in olivine ranges from 89.0% to 90.8%, and the enstatite component in orthopyroxene ranges from 89.4% to 91.1%, suggesting that they are mantle peridotites. Two different protoliths are identified for the serpentinites based on the bulk-rock compositions and spinel chemistry: abyssal peridotites and forearc mantle peridotites. Hydrated abyssal peridotites are voluminous and occur in ophiolite complexes in the northern terranes (Puerto Plata Complex and the northern part of the Rio San Juan Complex) and in serpentinite melanges in the central part of the Rio San Juan Complex. The serpentinite melanges contain fragments of high-pressure–low-temperature rocks and are interpreted to be tectonic melanges, representing part of a serpentinite subduction channel. The serpentinites show moderate Al/Si weight ratios (0.026–0.081) in bulk rocks and moderate Cr# (atomic ratio of Cr/[Cr + Al] = 0.20–0.55) in spinel. Hydrated forearc mantle peridotites occur along major strike-slip faults, the Camu fault zone, and the Septentrional fault zone. They show low bulk-rock Al/Si weight ratios (up to 0.021), high concentration in Ir-group platinum group elements (13.1–24.6 ppb total), and high Cr# (0.48–0.67) in spinel. Raman spectroscopy and X-ray powder diffraction indicate that lizardite is the predominant serpentine species. The absence of antigorite suggests that these serpentinites were derived from a shallow depth (<

/pdf/contrasting-origins-of-serpentinites-in-a-subduction-complex-1yffoxaulv.pdf

Contrasting origins of serpentinites in a subduction complex, northern Dominican Republic

Ferritchromite is rarely reported in forearc mantle peridotites. This contribution describes ferritchromite alteration and zoned Cr-spinel in serpentinites from the Rio San Juan Complex in the Dominican Republic. These rocks originated from the forearc mantle and protruded along lithosphere-scale faults in the mid Eocene. The cores of the Cr-spinel grains have Cr# ratios [i.e. atomic Cr/(Cr + Al)] between 0.48 and 0.66; such values are relatively high and are considered to represent primary compositions. Relatively high Zn contents in the grain cores (0.460.95 wt.% ZnO) are also thought to be primary; they reflect exceptionally cool conditions in the northern Caribbean forearc mantle. A progressive change in the zoning of Cr-spinel is recorded in the samples. Weakly zoned grains of Cr-spinel have rims with lower Mg# ratios [i.e. atomic Mg/(Mg + Fe 2+ )] and slightly higher Cr# ratios than the cores. More strongly zoned grains of Cr-spinel, in addition to low Mg# and high Cr# in their rims, have a marked increase in Fe 3+ # [i.e. Fe 3+ /(Fe 3+ + Al + Cr)] of up to 0.35 in their rims and are partially coated by Mg-rich chlorite. All grains show core-to-rim decreases in their Zn content and increases in Ti, Mn and V. The association with Mg-rich chlorite and the compositional zoning are reminiscent of those reported for ferritchromite. Ferritchromite (with Fe 3+ # >0.5) is common in ultramafic rocks in amphibolite-grade terranes; however, the serpentinite samples described herein show little evidence of high-grade metamorphism. The lowtemperature serpentine-group mineral lizardite is dominant and high-temperature antigorite is either very rare or absent; other high-temperature minerals, such as talc, tremolite and cummingtonite, are trace constituents. The observed zoning in the Cr-spinel is thought to represent ‘immature’ ferritchromite, probably formed in response to a short-lived thermal event. This event appears to have been on too short a timescale to produce either proper ferritchromite or significant quantities of high-temperature minerals. It may be related to the emplacement of the nearby Rio Boba Intrusion, or the upward protrusion of the serpentinites along the lithosphere-scale Septentrional fault zone from the base of the mantle wedge through its hotter interior. We suggest that such alteration is rare in forearc serpentinites because they are not commonly heated during exhumation along the plane of subduction. This work demonstrates that Cr-spinel compositions can be modified by relatively low-grade metamorphism.

Zoned Cr-spinel and ferritchromite alteration in forearc mantle serpentinites of the Rio San Juan Complex,

Corundum-bearing garnet peridotite from northern Dominican Republic: A metamorphic product of an arc cumulate in the Caribbean subduction zone

Ferritchromite is rarely reported in forearc mantle peridotites. This contribution describes ferritchromite alteration and zoned Cr-spinel in serpentinites from the Rio San Juan Complex in the Dominican Republic. These rocks originated from the forearc mantle and protruded along lithosphere-scale faults in the mid Eocene. The cores of the Cr-spinel grains have Cr # ratios [i.e. atomic Cr/(Cr + Al)] between 0.48 and 0.66; such values are relatively high and are considered to represent primary compositions. Relatively high Zn contents in the grain cores (0.46–0.95 wt.% ZnO) are also thought to be primary; they reflect exceptionally cool conditions in the northern Caribbean forearc mantle. A progressive change in the zoning of Cr-spinel is recorded in the samples. Weakly zoned grains of Cr-spinel have rims with lower Mg # ratios [i.e. atomic Mg/(Mg + Fe 2+ )] and slightly higher Cr # ratios than the cores. More strongly zoned grains of Cr-spinel, in addition to low Mg # and high Cr # in their rims, have a marked increase in Fe 3 + # [i.e. Fe 3+ /(Fe 3+ + Al + Cr)] of up to 0.35 in their rims and are partially coated by Mg-rich chlorite. All grains show core-to-rim decreases in their Zn content and increases in Ti, Mn and V. The association with Mg-rich chlorite and the compositional zoning are reminiscent of those reported for ferritchromite. Ferritchromite (with Fe 3 + # >0.5) is common in ultramafic rocks in amphibolite-grade terranes; however, the serpentinite samples described herein show little evidence of high-grade metamorphism. The low-temperature serpentine-group mineral lizardite is dominant and high-temperature antigorite is either very rare or absent; other high-temperature minerals, such as talc, tremolite and cummingtonite, are trace constituents. The observed zoning in the Cr-spinel is thought to represent ‘immature’ ferritchromite, probably formed in response to a short-lived thermal event. This event appears to have been on too short a timescale to produce either proper ferritchromite or significant quantities of high-temperature minerals. It may be related to the emplacement of the nearby Rio Boba Intrusion, or the upward protrusion of the serpentinites along the lithosphere-scale Septentrional fault zone from the base of the mantle wedge through its hotter interior. We suggest that such alteration is rare in forearc serpentinites because they are not commonly heated during exhumation along the plane of subduction. This work demonstrates that Cr-spinel compositions can be modified by relatively low-grade metamorphism.

/pdf/zoned-cr-spinel-and-ferritchromite-alteration-in-forearc-21lyb9s9kz.pdf

Zoned Cr-spinel and ferritchromite alteration in forearc mantle serpentinites of the Rio San Juan Complex, Dominican Republic

The Voisey’s Bay intrusion of the Mesoproterozoic Nain Plutonic Suite hosts a world-class magmatic Ni-Cu-Co sulfide deposit. Here we document the brittle and ductile structure of gneisses surrounding the intrusion using field data, borehole intersections, and subsurface structural data obtained from acoustic and optical televiewer surveys. We elucidate the role that wall-rock structure played on controlling the geometry of the intrusion and the formation of structural traps that host mineralization. The Voisey’s Bay intrusion consists at first order of two km-scale magma chambers, interpreted to represent an upper and lower chamber of the intrusion, that are connected by a system of dikes. Three types of gneiss, associated with the Paleoproterozoic Torngat orogen, form the wall rocks of the intrusion: Archean orthogneiss, Paleoproterozoic Tasiuyak paragneiss, and Paleoproterozoic enderbitic orthogneiss. Three phases of ductile deformation (D1–D3), predating intrusion emplacement, are recorded in the wall-rock gneisses. D1 produced the dominant S1 fabric, subparallel to original bedding (S) in the Tasiuyak gneiss. D2 shortening caused shallowly plunging, upright to moderately inclined, F2 folds in the enderbitic gneiss, the prototype of which was likely emplaced toward the end of D1. D3 ductile deformation resulted in steeply dipping shear zones, and flat lying to shallowly dipping C3 shear planes. At least two later phases of brittle deformation (D4 and D5) are present. D4 is a preemplacement event characterized by fractures with orientations similar to D3 shear zones and shear planes. D5 faults and joints formed during syn- to postemplacement brittle deformation likely associated with E-W brittle sinistral transtension associated with the Mesoproterozoic Gardar-Voisey’s Bay fault zone. The geometry of the Voisey’s Bay intrusion and its mineralization was controlled by pre- to synemplacement brittle structures and preemplacement ductile structures, which acted as passive wall-rock anisotropies. Mineralization hosted within Tasiuyak gneiss is controlled by the intersection of a dike with S1/S, D3 shear zones and D4 fractures. Where the intrusion occurs within enderbitic gneiss, mineralization locally coincides with shallow fabrics associated with F2 fold closures. The margins of the Voisey’s Bay dikes are controlled by steeply and shallowly dipping D4 fractures. Changes in dike thickness, which control the locations of mineralization, can be explained by thermomechanical erosion of the walls of the dike, which occurs by thermal expansion and contraction of wall rock due to repeated magma pulses, and the mechanical spalling of this wall rock along anisotropies. The strong brittle structural control on emplacement, along with previous geothermobarometry, is consistent with a mid-crustal (9–11 km) depth of emplacement. Lithosphere-scale E-W–trending D5 faults may have acted as conduits for the crustal-scale transport of primitive Voisey’s Bay magmas from their source, rather than an older orogenic suture zone.

B M Saumur

Papers

Contrasting origins of serpentinites in a subduction complex, northern Dominican Republic

Zoned Cr-spinel and ferritchromite alteration in forearc mantle serpentinites of the Rio San Juan Complex,

Corundum-bearing garnet peridotite from northern Dominican Republic: A metamorphic product of an arc cumulate in the Caribbean subduction zone

Zoned Cr-spinel and ferritchromite alteration in forearc mantle serpentinites of the Rio San Juan Complex, Dominican Republic

Wall-Rock Structural Controls on the Genesis of the Voisey’s Bay Intrusion and its Ni-Cu-Co Magmatic Sulfide Mineralization (Labrador, Canada)