The Cuillin layered igneous complex—evidence for multiple intrusion and former presence of a picritic liquid

Abstract

SYNOPSISMuch of the western Cuillin is formed by part of a funnel-shaped intrusion comprising layered ultrabasic rocks which range upwards from dunite to peridotite to allivalite (feldspar-olivine cumulate). The funnel-shaped intrusion has a marginal border group on which allivalites are banked. Recent mapping indicates that a peridotite mass previously regarded as a sill, is part of this layered sequence, and that inner, ultrabasic cumulates to the E are merely a downthrown portion of the same funnel-shaped mass. The eastern rocks were cut by a later pyroxene-rich magma from which feldspar-olivine-pyroxene cumulates were formed. In the lower dunite-peridotite-allivalite series (Unit 1), cumulus olivine and feldspar range from Fo 9 0 -8 6 and An 91 _ 85 , respectively. In the upper unit (Unit 2) of feldspar-olivine-pyroxene cumulates cryptic variation is more complex; feldspar cores are An 84 -83 , and olivines range from Fo 7 7 -6 7 .The high level of the Cuillin complex suggests that there was insufficient room for a layered basic intrusion some 10,000 metres thick. Evidence is presented for successive pulses of ultrabasic magma, indicating that the complex as a whole comprises intrusions of tholeiitic affinity ranging broadly with time from ultrabasic to gabbroic, rather than a tectonically disrupted single, layered basic intrusion.

Petrology of the Indian eucrite Piplia Kalan

“…Clouding of minerals similar to that displayed by lithologies A and B is found among mctamorphosed Tertiary lavas sampled -0.5 m from the contact with the Cuillin layered igneous complex on the Isle of Skye (Hutchison and Bevan, 1977). The centers of pyroxenes in these metabasalts commonly display abundant small inclusions along preferred crystallographic planes.…”

Layering in the Gars-bheinn ultrabasic sill, Isle of Skye: a new interpretation, and its implications

“…According to him this sequence was: first, Gars-bheinn type gabbro was intruded into cold basalts, then came two ring intrusions of eucrite (Ring Eucrite and Ghrunnda Eucrite), followed by the emplacement of the Sgurr Dubh peridotite and, finally, the intrusion of the 'Inner Layered Gabbro' (Layered Eucrite Intrusion) (Table 3). Gars-bheinn type gabbro outcrops at a higher structural level than Ring Eucrite (the contact is not exposed), and if the gabbro were earlier it ought to display the effects of hydrothermal activity present in the basalts and Weedon (1961) This work, and Hutchison and Bevan (1977) (Hutchison 1968), which also includes the Sgurr Dubh peridotite mass (Hutchison and Bevan 1977). ultrabasic sills.…”

“…In general, Gars-bheinn type gabbro was found to grade into gabbros with clouded feldspars, indicating that both groups of gabbros pre-date the emplacement of the ultrabasic rocks (Weedon 1961). Hutchison (1966a) questioned the reliability of clouding of feldspars as an indicator of age-relationships in the Cuillin, and suggested that the 'Clouded Feldspar Gabbros' (Weedon 1961) 'could post-date the emplacement of the Sgurr Dubh mass'. Further evidence for this has recently been found.…”

“…However, we prefer to be cautious, so in Figure 1 the rocks with clouded feldspar are designated 'affinity uncertain', unless observational evidence of their relationship is available. Weedon's (1961) interpretation of the intrusive sequence and identities of the intrusive units (Table 3) is compared with those outlined by Hutchison (1968) and Hutchison and Bevan (1977). The Ghrunnda Eucrite was shown to be allivalitic (White Allivalite) and part of a border group marginal to a layered ultrabasic sequence that includes layered allivalites and the peridotites and dunites of Sgurr Dubh.…”

The chilled margin of the Ben Buie layered gabbro, Isle of Mull

“…Brown (1956), on the basis of chemical and textural evidence, concluded that the parent magma of the Rhum ultrabasic layered intrusion was of basaltic composition, but Gibb (1976) has argued that an ultrabasic parentage would overcome the 'room' problem as well as helping to explain a number of the features of both this intrusion and of the Cuillin layered complex in Skye. Walker (1975), Hutchison (1968) and Hutchison and Bevan (1977) also have favoured an ultrabasic magma for the Cuillin complex but there is no chilled margin that may be used as evidence.…”