HR: 16:30h
AN: V44C-03 INVITED     [Abstracts]
TI: Geochemical Evidence from the Kohistan Complex for Differentiation of Garnet Granulitic lower Crust in Island Arcs by Dehydration Melting of Amphibole-bearing Plutonics: Implications for the Andesite Model of Continental Crustal Growth
AU: * Garrido, C J
EM: carlosg@ugr.es
AF: Dpto Mineralogia y Petrologia, Universidad de Granada, Facultad de Ciencias, Fuentenueva s/n, Granada, 18002 Spain
AU: Bodinier, J
EM: Jean.Louis.Bodinier@dstu.univ-montp2.fr
AF: ISTEEM, Laboratoire de Tectonophysique, CNRS & Universite de Montpellier II., Place Eugene Bataillon s/n, Montpellier, 34095 France
AU: Burg, J
EM: jean-pierre.burg@erdw.ethz.ch
AF: Geologisches Institut, ETH-Zentrum, Sonneggstrasse 5, Zurich, CH-8092 Switzerland
AU: Zeilinger, G
EM: zeilinger@geo.unibe.ch
AF: Institute of Geological Sciences, Univ. Bern, Baltzerstrasse 1-3, Bern, CH-3012 Switzerland
AU: Hussain, S S
EM: sshahid_hussain@hotmail.com
AF: Pakistan Museum of Natural History, Garden Avenue, Shakarparian., Islamabad, 4400 Pakistan
AU: Dawood, H
EM:
AF: Pakistan Museum of Natural History, Garden Avenue, Shakarparian., Islamabad, 4400 Pakistan
AU: Gervilla, F
EM: gervilla@ugr.es
AF: Dpto Mineralogia y Petrologia, Universidad de Granada, Facultad de Ciencias, Fuentenueva s/n, Granada, 18002 Spain
AB: We report a geochemical study of the Jijal and Sarangar complexes constituting the lower crust of the Mesozoic Kohistan paleo-island arc (N. Pakistan). The Jijal complex is composed of basal peridotites topped by a gabbroic section made up of mafic garnet granulite-with minor lenses of garnet hornblendite and granite-grading up section to hornblende gabbronorite. The Sarangar complex is constituted by metagabbro. Sarangar gabbro and Jijal hornblende gabbronorite have melt-like, LREE-enriched REE patterns similar to those of island arc basalts. These rocks and Jijal garnet granulite define altogether negative covariations of La_N, Yb_N and (La/Sm)_N with Eu* (=2xEu_N/Sm_N+Gd_N; N= chondrite normalized), and positive covariations of (Yb/Gd)_N with Eu*. REE modeling indicates that these covariations cannot be accounted for by high-pressure crystal fractionation of hydrous primitive or derivative andesites. They are consistent with formation of garnet granulites as plagioclase-garnet assemblages with variable trapped melt fractions via either high-pressure crystallization of primitive island arc basalts or dehydration melting of hornblende gabbronorite, providing that the amount of segregated or restitic garnet was low (< 5 wt.%). Field, petrographic, geochemical and experimental evidence is more consistent with formation of Jijal garnet granulite by dehydration melting of Jijal hornblende gabbronorite. Similarly, Jijal garnet-bearing hornblendite lenses were most likely generated by coeval dehydration melting of hornblendites. Furthermore, melting models and geochronological data point to intrusive leucogranites in the overlying Metaplutonic complex as the melts generated by dehydration melting of the plutonic protoliths of Jijal garnet-bearing restites. Consistently with the metamorphic evolution of the Kohistan lower arc crust, dehydration melting occurred at the mature stage of this island arc when shallower hornblende-bearing plutonics were buried to depth exceeding 25-30 km and heated at temperatures above ca. 900 ^oC. Available experimental data on dehydration melting of amphibolitic sources imply that thickening of oceanic arcs at depth > 30 km (ca 1.0 GPa,), together with the high geothermal regime now postulatedfor lower island arc crust, should cause dehydration melting of amphibole-bearing plutonic rocks generating dense garnet granulitic roots in island arcs. Dehydration melting of hornblende-bearing plutonics may hence be a common intracrustal chemical and physical differentiation process of island arcs-and a natural consequence of their maturation-leading to the addition of granitic partial melts to the middle-upper arc crust and formation of dense, unstable garnet granulite roots in the lower arc crust. Addition of LREE- and silica-rich melts produced by this process to the middle-upper island arc crust may drive its basaltic composition toward that of andesite affording a plausible solution to the arc paradox of formation of andesitic Continental-like Crust in island arc settings.
DE: 1020 Composition of the continental crust
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005