Genesis of felsic plutonic magmas and their igneous enclaves: the cobaw batholith of southeastern Australia

TY - JOUR

T1 - Genesis of felsic plutonic magmas and their igneous enclaves

T2 - the cobaw batholith of southeastern Australia

AU - Clemens, John D.

AU - Maas, Roland

AU - Waight, Tod Earle

AU - Kunneke, Luke K.

PY - 2016/5

Y1 - 2016/5

N2 - The Late Devonian postorogenic Cobaw batholith, in southeastern Australia, is an oval, east-west-orientated, terrane-stitching lopolith that intruded low-grade metaturbidites. The initial intrusion (at 370 Ma) was the small, hypabyssal, S-type Rainy Creek Rhyolite (RCR). At 369 Ma, the foliated S-type Pyalong pluton was emplaced, apparently along an east-west-orientated fracture zone. Around 367 Ma, the main I-type Baynton pluton intruded as numerous shallow-dipping sheets. The last plutonic event was the intrusion of the broad, thin, flat-lying, and crosscutting sheet of the I-type Beauvallet pluton. The Cobaw plutons had independent origins, with magmas derived from contrasting, internally heterogeneous source rocks. For both the RCR and the Pyalong pluton, the sources were old metasedimentary rocks, while the magmas for the Baynton and Beauvallet plutons formed through partial melting of metadacitic to meta-andesitic rocks, probably with an admixture of immature volcaniclastic graywackes. The source rocks for the Baynton magmas were more crustally evolved than those for the Beauvallet magmas. Heterogeneity in the Baynton pluton did not result from crystal fractionation or magma mixing, despite the relatively high abundance of igneous-textured microgranular enclaves (MEs). The MEs show neither chemical nor isotope mixing trends with each other or with the host magmas. Variations in the Baynton magmas were derived from the heterogeneity of the source terrane, with individual magma batches formed from mixtures of metaigneous and metasedimentary rocks. Baynton MEs are isotopically less evolved than their host rocks. Their chemistry, textures, and field relations suggest that they represent small volumes of hybrid magmas formed through near-source mixing between crustal melts and mantle-derived mafic magmas. Further modification occurred through melt loss and ingestion of host crystals as the MEs were deformed, in the plastic state, during magmatic flow of their hosts.

AB - The Late Devonian postorogenic Cobaw batholith, in southeastern Australia, is an oval, east-west-orientated, terrane-stitching lopolith that intruded low-grade metaturbidites. The initial intrusion (at 370 Ma) was the small, hypabyssal, S-type Rainy Creek Rhyolite (RCR). At 369 Ma, the foliated S-type Pyalong pluton was emplaced, apparently along an east-west-orientated fracture zone. Around 367 Ma, the main I-type Baynton pluton intruded as numerous shallow-dipping sheets. The last plutonic event was the intrusion of the broad, thin, flat-lying, and crosscutting sheet of the I-type Beauvallet pluton. The Cobaw plutons had independent origins, with magmas derived from contrasting, internally heterogeneous source rocks. For both the RCR and the Pyalong pluton, the sources were old metasedimentary rocks, while the magmas for the Baynton and Beauvallet plutons formed through partial melting of metadacitic to meta-andesitic rocks, probably with an admixture of immature volcaniclastic graywackes. The source rocks for the Baynton magmas were more crustally evolved than those for the Beauvallet magmas. Heterogeneity in the Baynton pluton did not result from crystal fractionation or magma mixing, despite the relatively high abundance of igneous-textured microgranular enclaves (MEs). The MEs show neither chemical nor isotope mixing trends with each other or with the host magmas. Variations in the Baynton magmas were derived from the heterogeneity of the source terrane, with individual magma batches formed from mixtures of metaigneous and metasedimentary rocks. Baynton MEs are isotopically less evolved than their host rocks. Their chemistry, textures, and field relations suggest that they represent small volumes of hybrid magmas formed through near-source mixing between crustal melts and mantle-derived mafic magmas. Further modification occurred through melt loss and ingestion of host crystals as the MEs were deformed, in the plastic state, during magmatic flow of their hosts.

U2 - 10.1086/685509

DO - 10.1086/685509

M3 - Journal article

SN - 0022-1376

VL - 124

SP - 293

EP - 311

JO - Journal of Geology

JF - Journal of Geology

IS - 3

ER -