Oral
presentation at the 33rd International Geological Congress, Oslo, Norway,
6-14 August, 2008
Session: MPN-03 Mineral replacement and mass transfer
in hydrothermal systems: From the nanoscale to the megascale
Abstract
Title
K- and Si-metasomatism created K-feldspar megacrystic granite in the outer
shell of the Vrådal pluton, Telemark, southern Norway
Arthur G. Sylvester
Earth Sciences Department
University
of California Santa Barbara
email: sylvester@geol.ucsb.edu
Lorence G. Collins
Department of Geological
Sciences
California State
University Northridge
email: lorencecollins@gmail.com
Abstract Text
Vrådal pluton (967 ± 4 ma) is one of several undeformed Sveconorwegian granitic
plutons that intruded the South Telemark Gneiss (ca. 1210 ma) in southern Norway
(Sylvester, 1964; 1998; Andersen et al., 2001; 2007). It is a nearly circular pluton, 6 km in diameter, with a
central core, about 4 km in diameter, of white and pink granodiorite surrounded
originally by a 750-1000 m-wide shell of mafic granodiorite. K-feldspar
megacrysts, 2- 5 cm long and strongly oriented concentric to the pluton margin,
grew metasomatically in the mafic granodiorite, converting it into megacrystic
granite. Irregular masses of hybridized diabase comprise 25% of the pluton.
Contacts among the rock facies are gradational over meters to tens of meters.
Modes of the granitic rocks are as follows: core granodiorite has 40% plagioclase,
35% microcline, 20% quartz, and 5 % biotite; outer shell megacrystic granite
has 30% plagioclase, 35% microcline, 25% quartz, and 5% biotite; and mafic
granodiorite has 45% plagioclase, 10% microcline, 15 % quartz, 10% hornblende,
and 20% biotite and chlorite. Anhedral titanite and magnetite are so abundant
that the pluton constitutes a significant regional magnetic anomaly (Sylvester,
1998). Thin section petrography reveals
that K-feldspar slightly to completely replaced
plagioclase in the outer shell mafic granodiorite, commonly outward from the
plagioclase cores. Myrmekite is locally present. Hornblende and biotite were
replaced by quartz, so that the quartz mode increased
from 15 to 25%, but some biotite (5%) remains in the megacrystic granite. Ghost
plagioclase zoning is preserved in K-feldspar of partly replaced plagioclase
crystals. One plane of the microcline grid twinning typically parallels the
Carlsbad/albite twinning of the plagioclase. Some megacrysts have secondary
oligoclase mantles. Relic plagioclase in the megacrystic granite is partly to
intensely micro-fractured, caused, we infer, by synplutonic cataclastic
deformation when the pluton's core continued to rise diapirically relative to
its almost solidified outer shell.The microfractures were loci of, and pathways
for, pervasive fluids containing Si that wholly replaced hornblende as quartz
and K that partially and wholly replaced plagioclase to form K-feldspar.
Because the K-feldspar megacrysts are 2-4 times larger than the replaced plagioclase,
they subsequently grew beyond the boundaries of the original plagioclase and
enclosed groundmass minerals. They
inherited their concentric preferred orientation from the magmatic flow fabric
of primary plagioclase in the mafic granodiorite. Some K for K-feldspar megacrysts came from
the breakdown of biotite in the
mafic granodiorite, but much K must have come from breakdown
of older, felsic, metaigneous rocks in the pluton's source region. Ca released from replaced plagioclase went into titanite.
Excess Fe was precipitated in magnetite. Excess Ca and Mg were subtracted from
the system.