Melting events in the Rum layered intrusion

Julien Leuthold

Research output: Contribution to conferenceConference Abstract


The Rum Eastern Layered Intrusion (RELI) Unit 9 is composed of peridotite with poikilitic Plg and Cpx, troctolite with interstitial Cpx and eqigranular gabbro. Gabbro enclaves occur within troctolite. Cpx rims are poorly developped in layered peridotite and troctolite and bulk rock analyses display distinct Eu positive anomalies, evidencing loss of interstitial liquid. Cpx rims are Cr-poor, REE-rich and display low La/Lu (strongly incompatible/moderatly incompatible elements) ratio in regard to associated cores. Troctolite is overlain by gabbro, separated by a wavy horizon. Gabbro Cpx show discrete reverse Cr and REE zoning and a constant high La/Lu ratio. Troctolite interstitial Cpx and gabbro equigranular Cpx progressively get oikocrystic towards the northern edge of an intrusive peridotite sill, forming poikilitic gabbro, with Cpx oikocrysts. Cpx Cr-rich (~1.1 wt% Cr2O3), REE-moderate, high La/Lu anhedral core enclose randomly oriented small (up to 0.5 mm) Ol and Plg inclusions. They are overgrown by a Cr-moderate (~0.7 wt% Cr2O3), REE-poor, high La/Lu anhedral core. Cpx dissolution occurs along inclusions. Poikilitic rims are Cr-poor (~0.2 wt% Cr2O3), REE-rich and display low La/Lu ratio. They enclose large (1 mm) oriented Plg and Ol crystals. The rim thickness increases from the Unit 9 base to the top. The Cpx rim crystallization is synchronous with cumulate pile compaction.
Equilibrium liquids were estimated using Wood and Blundy (1997) partition coefficients. The liquid at the origin of the peridotite, troctolite and poikilitic gabbro Cpx cores display a higher La/Lu ratio than the corresponding rims and the picritic dikes. The high La/Lu ratio of the liquids in equilibrium with the Cpx cores is interpreted as a signature of low degree partial melting. Cpx cores is best explained by 5% partial fractionated melting of a gabbroic assemblage (RELI interstitial crystals or gabbro). Gabbro enclaves within troctolite are relics. The multiple generations of Cpx are witnesses of succesive melting episodes. Cpx rims are best explained by partial melting (5%) of a gabbroic assemblage, mix with picritic liquid (30-50%) and subsequent Rayleigh fractional crystallization (F = 1-0.7).
Known RELI liquids (Upton et al., 2002) fail to model crystallization of the Unit 9 Cpx cores by simple fractional crystallization. We suggest that the Unit 9 gabbro crystallized from aggregated drops from a partially molten gabbroic assemblage deeper in the RELI. Subsequent intrusion of peridotite sills induced partial melting of the Unit 9 gabbro. Once porosity was sufficient, differentiating liquids from the peridotite percolated throughout the crystal mush pile, mixing with interstitial liquid and crystallizing the interstitial crystals. Thus the composition of the percolating liquid and the final metasomatized cumulate differ from simple fractionation products along the Rum liquid line of descent.

Wood, B.J., Blundy, J.D. (1997) Contrib. Mineral. Petrol. 129, 166-181.
Upton, B.G.J et al. (2002) Geol. Mag., 139, 437-452.
Original languageEnglish
Publication statusIn preparation - 2013


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