TY - JOUR
T1 - Examples of convective fractionation in high-temperature granites from the Lachlan Fold Belt
AU - Wyborn, D.
AU - Chappell, B. W.
AU - James, M.
PY - 2001
Y1 - 2001
N2 - Igneous rocks derived from high-temperature, crystal-poor magmas of intermediate potassic composition are widespread in the central Lachlan Fold Belt, and have been assigned to the Boggy Plain Supersulte. These rocks range in composition from 45 to 78% SiO2, with a marked paucity of examples in the range 65-70% SiO2, the composition dominant in most other granites of the Lachlan Fold Belt. Evidence is presented from two units of the Boggy Plain Supersulte, the Boggy Plain zoned pluton and the Nallawa complex, to demonstrate that these high-temperature magmas solidified under a regime of convective fractionation. By this process, a magma body solidified from margin to centre as the zone of solidification moved progressively inwards. High-temperature near-liquidus minerals with a certain proportion of trapped interstitial differentiated melt, separated from the buoyant differentiated melt during solidification. In most cases much of this differentiated melt buoyantly rose to the top of the magma chamber to form felsic sheets that overly the solidifying main magma chamber beneath. Some of these felsic tops erupted as volcanic rocks, but they mainly form extensive high-level intrusive bodies, the largest being the granitic part of the Yeoval complex, with an area of over 200 km2. Back-mixing of fractionated melt into the main magma chamber progressively changed the composition of the main melt, resulting in highly zoned plutons. In the more felsic part of the Boggy Plain zoned pluton back-mixing was dominant, if not exclusive, forming an intrusive body cryptically zoned from 63% SIO2 on the margin to 72% SIO2 in the core. It is suggested that tonalitic bodies do not generally crystallise through convective fractionation because the differentiated melt is volumetrically small and totally trapped within the interstitial space: back-mixing is excluded and homogeneous plutons with essentially the composition of the parental melt are formed.
AB - Igneous rocks derived from high-temperature, crystal-poor magmas of intermediate potassic composition are widespread in the central Lachlan Fold Belt, and have been assigned to the Boggy Plain Supersulte. These rocks range in composition from 45 to 78% SiO2, with a marked paucity of examples in the range 65-70% SiO2, the composition dominant in most other granites of the Lachlan Fold Belt. Evidence is presented from two units of the Boggy Plain Supersulte, the Boggy Plain zoned pluton and the Nallawa complex, to demonstrate that these high-temperature magmas solidified under a regime of convective fractionation. By this process, a magma body solidified from margin to centre as the zone of solidification moved progressively inwards. High-temperature near-liquidus minerals with a certain proportion of trapped interstitial differentiated melt, separated from the buoyant differentiated melt during solidification. In most cases much of this differentiated melt buoyantly rose to the top of the magma chamber to form felsic sheets that overly the solidifying main magma chamber beneath. Some of these felsic tops erupted as volcanic rocks, but they mainly form extensive high-level intrusive bodies, the largest being the granitic part of the Yeoval complex, with an area of over 200 km2. Back-mixing of fractionated melt into the main magma chamber progressively changed the composition of the main melt, resulting in highly zoned plutons. In the more felsic part of the Boggy Plain zoned pluton back-mixing was dominant, if not exclusive, forming an intrusive body cryptically zoned from 63% SIO2 on the margin to 72% SIO2 in the core. It is suggested that tonalitic bodies do not generally crystallise through convective fractionation because the differentiated melt is volumetrically small and totally trapped within the interstitial space: back-mixing is excluded and homogeneous plutons with essentially the composition of the parental melt are formed.
KW - Boggy plain supersuite
KW - Convective fractionation
KW - I-type granite
KW - In situ crystallisation
KW - Lachlan Fold Belt
UR - http://www.scopus.com/inward/record.url?scp=0034902418&partnerID=8YFLogxK
U2 - 10.1046/j.1440-0952.2001.00877.x
DO - 10.1046/j.1440-0952.2001.00877.x
M3 - Article
AN - SCOPUS:0034902418
SN - 0812-0099
VL - 48
SP - 531
EP - 542
JO - Australian Journal of Earth Sciences
JF - Australian Journal of Earth Sciences
IS - 4
ER -