Elsevier

Chemical Geology

Volume 77, Issues 3–4, 15 November 1989, Pages 183-207
Chemical Geology

Mineral chemistry of ultramafic tectonites and ultramafic to gabbroic cumulates from the major oceanic basins and Northern Apennine ophiolites (Italy) — A comparison

https://doi.org/10.1016/0009-2541(89)90074-0Get rights and content

Abstract

The paper presents the mineralogy of upper-mantle tectonites and associated lower-crustal ultramafic and mafic cumulates from the Jurassic Northern Apennine ophiolites (NAO) in a comparison assay with equivalent rocks from the major oceanic basins (MOB: East Pacific Rise and Atlantic fracture zones, Legs 37 and 45). Among the mantle rocks both tectonite lherzolites and harzburgites have been so far widely found in MOB whereas lherzolites largely prevail in NAO. The Mg# [Mg# = Mg(Mg + Fe2+)] range in mafic minerals in NAO tectonites is restricted to high values (Ol: 88–90; Opx: 90–92; Cpx: 91–93). Cr-spinels show large variation of the Cr# [Cr(Cr + Al)] ratio found in spinels from lherzolites and very rare Cpx-bearing harzburgites (Cr# 10 and 15, respectively). The varied compositions largely reflect the relative greater degree of partial melting that affected MOB tectonites compared to NAO tectonites.

From a restored stratigraphic log of the NAO cumulates the order of appearance of the cumulus phases both in NAO and MOB cumulates is: Ol + Chr, Pl, Cpx, Opx, Fe/1bTi-oxides, Ap. The sequence gave rise to the following main lithotypes: dunites, troctolites, minor wehrlites, Ol-gabbros, gabbronorites, ferrogabbros and ferrodiorites. The olivine from early ultramafic cumulates has high Fo content (Fo90 for NAO, Fo91 for MOB) with a NiO content ranging from 0.21 to 0.31 wt.%. Cr-diopside generally shows higher TiO2 than found in tectonite peridotites. The coexisting spinel is typically a Ti-chromite with high Cr# (43–60). The most Mg-rich olivine coexisting with An-rich plagioclase in the most primitive cumulates are Fo88 and An72 and were in equilibrium with a basaltic melt. These facts coupled with the lack of intense plastic deformation in the early cumulates allow us to exclude a high-pressure fractionation system. Formation of very little volume percent of amphibole indicates a very low H2O activity in the magma.

Starting from the early fractionating cumulus phases a recalculated primary magma composition is proposed. From these calculations it is concluded that NAO cumulates derived from a tholeiitic basalt liquid similar to primitive MOB liquid except for a higher Na2O content. In particular present-day analogues are found among high Na2O mid-ocean ridge basalts from the Mid-Cayman Rise and Southwest Indian Ridge. The composition of calculated primary magmas indicated for NAO: (1) a much lower degree of melting than average MOB, also sustained by evident differences in residual peridotites; (2) a crustal thickness of ∼ 2–3 km, also supported by the reconstructed maximum thickness of crustal sequences of 2 km; (3) a spreading axis with an average water depth of ∼ 5 km. By analogy with the Mid-Cayman Rise and Southwest Indian Ridge, half spreading rates of <1 cm a−1 are suggested. Picritic parental magma can be excluded for NAO. We propose an Atlantic/Indian-type or Mid-Cayman Rise rift for the origin of NAO. This ridge would be cut by several fracture zones explaining the highly dismembering of the ophiolites sequences, the low partial melting rate involved and the Na2O enrichment character of the cumulates and related lavas.

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