Fractional crystallization, impregnation and sulphide saturation recorded in Mesozoic arc-related cumulates at King Mountain, Cache Creek Ophiolite, Northern British Columbia.
Abstract
The Cache creek terrane extends from southern B.C. to the Yukon. It accreted to North America at 175Ma and is composed of Paleozoic seamounts, Mesozoic oceanic arcs and mantle rocks. Mantle harzburgite massifs represent intra-oceanic core-complexes. Mantle rocks are cut by gabbroic dykes and overlain by chert, lava, dismembered hypabyssal complexes and rare cumulates. At King Mountain, gabbronorites are in tectonic contact with subjacent peridotite. Other crustal relics exposed nearby include sheeted hypabyssal intrusions and volcanics that range from depleted arc tholeiites to boninites. The King Mountain cumulates are rhythmically layered, foliated gabbronorites with 5% oxides and minor interstitial hornblende that yields temperatures of 652-759oC. Cumulates may show evidence of compaction-related flattening and intra-cumulate shear (boudins, fold noses). A 300m thick continuous section records two fractional crystallization cycles, whole rock mg# varying from 60 to 35 in the 1st cycle and from 52 to 30 in the 2nd. Cumulates formed during passage of evolved multiply-saturated magmas derived from a deeper chamber towards the surface. Inverse trace element models show that the gabbronorite cumulates are compositionally akin to boninites. The lowest-mg# rocks in the differentiation cycles are rusty 10cm-1m interbeds with abundant magnetite+ ilmenite ( 10-15%), high sulphide contents ( 5-10%, pyrrhotite and chalcopyrite) and high V contents (<1200ppm). These are interpreted to record episodic co-accumulation of Fe-Ti-oxides, with the decrease in melt FeO-content triggering sulphide immiscibility. Hornblendite and hornblende tonalite veins are locally transposed into the layered cumulates, forming flaser gabbros with 5-50% cm-scale lensoid hornblendite that impregnates and replaces the foliated gabbro-norite; greatly increasing REE contents. Amphibole oikocrysts show evidence of internal deformation and record temperatures of 753-804 oC.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.V31D..04B
- Keywords:
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- 1020 Composition of the continental crust;
- GEOCHEMISTRYDE: 1021 Composition of the oceanic crust;
- GEOCHEMISTRYDE: 1031 Subduction zone processes;
- GEOCHEMISTRYDE: 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY