Endogenous growth in channelized komatiite lava flows: evidence from spinifex-textured sills at Pyke Hill and Serpentine Mountain,Western Abitibi Greenstone Belt,Northeastern Ontario,Canada |
| |
| Authors: | Michel G Houlé Sonia Préfontaine Anthony D Fowler Harold L Gibson |
| |
| Institution: | (1) Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, 935 Ramsey Lake Road, Sudbury, Ontario, Canada, P3E 2C6;(2) Ottawa-Carleton Geoscience Centre, University of Ottawa, 140 Louis Pasteur, Ottawa, Ontario, Canada, K1N 6N5;(3) Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, Ontario, Canada, P3E 6B5 |
| |
| Abstract: | Spinifex-textured sills (i.e., veins) characterized by komatiitic magmas that have intruded their own volcanic-piles have
long been recognized. For instance, in the early 1970s, Pyke and coworkers, in their classic work at Pyke Hill in Munro Township,
noted that not all spinifex-bearing ultramafic rocks formed as lava flows, rather some were clearly emplaced as small dikes
and sills. Several hypotheses have been proposed to explain spinifex-textured sills: intrusion into a cold host, filter pressing,
or drainage of residual liquid. However, these do not satisfactorily explain the phenomenon. Field and petrographic observations
at Pyke Hill and Serpentine Mountain demonstrate that spinifex-bearing komatiite sills and dikes were emplaced during channel
inflation processes when new magma was intruded into a cooler, semi-consolidated but permeable cumulate material. Komatiitic
liquids were intruded into the olivine cumulate rocks near the boundary between the spinifex and the cumulate zones of well-organized
to organized komatiite flows. Spinifex-textured sills are generally tabular in morphology, stacked one above another, with
curviplanar contacts sub-parallel to stratigraphy. Some sills exhibit complex digitated apophyses. Thinner sills typically
have a random olivine spinifex texture similar, though generally composed of coarser crystals, to that of komatiite lava flows.
Thicker sills exhibit more complex organization of their constituent crystals characterized by zones of random olivine spinifex,
overlying zones of organized coarse spinifex crystals similar to those found in lava flows. They have striking coarse dendritic
spinifex zones composed of very large olivine crystals, up to several centimetres long and up to 1 cm wide that are not observed
in lava flows. Typically, at the sill margins, the cumulate material of the host flow is composed of euhedral to subhedral
olivine crystals that are larger than those distal to the contact. Many of these margin-crystals have either concentric overgrowth
shells or dendritic olivine overgrowths that grew from the cumulate-sill contact toward the sill interior. The dendrites grew
on pre-existing olivine cumulate at the contact in response to a sharp temperature gradient imposed by the intrusion of hot
material, whereas the concentric overgrowths formed as new melt percolated into the unconsolidated groundmass of the host-flow
cumulate material. Spinifex-textured sills and dikes occur in well-organized to organized flows that are interpreted to have
formed by “breakouts” above and peripheral to lava pathways (channels/conduits) as a result of inflation that accompanied
voluminous komatiitic eruptions responsible for the construction and channelization of komatiitic flow fields. The spinifex-textured
dikes and sills represent komatiitic lava that was originally emplaced into the channel roof during periods of episodic inflation
that resulted in lava breakouts and was subsequently trapped in the “roof rocks” during periods of channel deflation. Accordingly,
the occurrence of spinifex-textured sills and dikes may indicate proximity to, and aid in the identification and delineation
of lava channel-ways that could potentially host Ni–Cu–(PGE) mineralization within komatiitic lava flow-fields. |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
