Petrogenesis of Ultramafic Metamorphic Rocks from the 3800 Ma Isua Supracrustal Belt, West Greenland

• Published in: Journal of Petrology
• Main Authors: DYMEK, ROBERT F., BROTHERS, SARA C., SCHIFFRIES, CRAIG M.
• Format: Text
• Language: English
• Published: Oxford University Press 1988
• Subjects: Articles; Greenland
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/29/6/1353; https://doi.org/10.1093/petrology/29.6.1353

Based on mineral assemblage, we distinguish four ultramafic rock types at Isua: (1) Peridotite (olivine + chlorite + tremolite � cummingtonite/anthophyllite + Cr-magnetite � magnesite � dolomite � biotite, or olivine + orthopyroxene + Al,Cr-spinelsol;ferritchromite + tremolite + chlorite); (2) Chlorite Schist (olivine + chlorite + tremolite + cummingtonite + Cr-magnetite);(3) Talc-Magnesite Schist (talc + magnesite � antigorite + magnetite); and (4) Serpentinite (antigonte + magnetite � talc � magnesite). Orthopyroxene-free peridotites occur throughout the Isua belt and record a two-stage metamorphic history: prograd metamorphism up to ∼600�C (at high X CO 2 ), similar to temperature conditions inferred from pelitic and iron-formation assemblages; and retrograde metamorphism to talc-magnesite schist or serpentinite near ∼450�C at lower values of X CO 2 Orthopyroxene-bearing peridotites are found at only one locality, as a lens within aluminous ‘garbenschiefer’ amphibolite. These rocks appear to record a three-stage history: prograde meta morphism in excess of ∼650�C, dunng which orthopyroxene and spinel formed; re-equilibration at about 560�C concomitant with growth of retrograde chlorite; and further retrogression to a talc magnesite assemblage. The earliest (‘higher temperature’) event preserved in these peridotites is not recognized in other Isua rock types. Based on chemical composition, we distinguish three classes of peridotites: (1) Group A, which has Ni>Cr, low Al 2 O 3 (∼3–5 wt. %), Si0 2 /(FeO+MgO)>0�4, and smoothly fractionated REE ranging from ∼1–20 � chondrites with La N /Yb N ∼5 and small negative Eu-anomalies; (2) Group B, which has Ni>Cr, lower Al 2 O 3 (∼1 wt. %), Si0 2 /(FeO+MgO)<0�4, and kinked REE patterns at ∼l–3� having flat light REE, fractionated heavy REE and small negative Eu-anomalies; and (3) Group G (the garbenschiefer-hosted orthopyroxene-bearing type), which has Cr>Ni, relatively high Al 2 O 3 -contents (∼6–8 wt. %) and low REE abundances (∼0�2–1 �), with light REE ≤ ...