TY - JOUR
T1 - Cr-spinel Seam Petrogenesis in the Rum Layered Suite, NW Scotland
T2 - Cumulate Assimilation and in situ Crystallization in a Deforming Crystal Mush
AU - O'Driscoll, Brian
AU - Emeleus, C. H.
AU - Donaldson, Colin Henry
AU - Daly, J. S.
PY - 2010
Y1 - 2010
N2 - Laterally extensive (∼2 mm thick) Cr-spinel seams in the Rum Layered Suite, NW Scotland, occur at the bases of several of the coupled peridotite–troctolite macro-rhythmic units that form the bulk of the eastern part of the intrusion. Detailed petrography, mineral chemical analyses and quantitative textural measurements of the rocks above and below two of these seams suggest that existing models for seam petrogenesis involving early crystallization and gravitational settling of Cr-spinel from a newly emplaced body of magma need to be reassessed. We argue for assimilation of troctolitic cumulate by a new influx of picrite magma at the crystal mush–magma interface, and subsequent in situ crystallization of the Cr-spinel seams. The bases of seams are characterized by Mg- and Al-rich Cr-spinel, with slightly more Fe-rich crystals toward the tops. These seams crystallized from a superheated hybrid magma generated by the initial assimilation and dissolution of the plagioclase-rich cumulate floor by the picrite. Coeval syn-magmatic deformation of the crystal mush at the unit boundary between peridotite and troctolite caused localized expulsion of Cr-spinel seed crystals several centimetres upward into the peridotite mush, resulting in the formation of overlying supra-seams as well as possibly developing a distinctive texture comprising chains of Cr-spinel around cumulus olivine crystals immediately above unit boundaries. The mineral compositional and textural evidence collected from the Rum Cr-spinel seams also indicates that they exhibit varying degrees of postcumulus chemical and textural equilibration.
AB - Laterally extensive (∼2 mm thick) Cr-spinel seams in the Rum Layered Suite, NW Scotland, occur at the bases of several of the coupled peridotite–troctolite macro-rhythmic units that form the bulk of the eastern part of the intrusion. Detailed petrography, mineral chemical analyses and quantitative textural measurements of the rocks above and below two of these seams suggest that existing models for seam petrogenesis involving early crystallization and gravitational settling of Cr-spinel from a newly emplaced body of magma need to be reassessed. We argue for assimilation of troctolitic cumulate by a new influx of picrite magma at the crystal mush–magma interface, and subsequent in situ crystallization of the Cr-spinel seams. The bases of seams are characterized by Mg- and Al-rich Cr-spinel, with slightly more Fe-rich crystals toward the tops. These seams crystallized from a superheated hybrid magma generated by the initial assimilation and dissolution of the plagioclase-rich cumulate floor by the picrite. Coeval syn-magmatic deformation of the crystal mush at the unit boundary between peridotite and troctolite caused localized expulsion of Cr-spinel seed crystals several centimetres upward into the peridotite mush, resulting in the formation of overlying supra-seams as well as possibly developing a distinctive texture comprising chains of Cr-spinel around cumulus olivine crystals immediately above unit boundaries. The mineral compositional and textural evidence collected from the Rum Cr-spinel seams also indicates that they exhibit varying degrees of postcumulus chemical and textural equilibration.
UR - http://www.scopus.com/inward/record.url?scp=77954004429&partnerID=8YFLogxK
U2 - 10.1093/petrology/egq013
DO - 10.1093/petrology/egq013
M3 - Article
SN - 0022-3530
VL - 51
SP - 1171
EP - 1201
JO - Journal of Petrology
JF - Journal of Petrology
IS - 6
ER -