Geochemistry and preliminary Sr-Nd isotopic data on the Neoproterozoic granitoids from the Bantoum area, west Cameroon: evidence for a derivation from a Paleoproterozoic to Archaean crust
The Bantoum area in west Cameroon is composed of migmatitic gneisses associated with parallel strips of amphibolites,quartz-monzonites,biotite-granites, two-mica leucogranites and granitic dikes.Quartz-monzonites are metaluminous (A/CNK=0.8-0.9)I-type,biotite-granites are peraluminous (A/CNK=1.0-1.1...
Main Authors: | , , , |
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Format: | Report |
Language: | English |
Published: |
Graduate School of Science and Technology, Niigata University
2003
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Subjects: | |
Online Access: | https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=3129 http://id.nii.ac.jp/1291/00003129/ https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=3129&item_no=1&attribute_id=18&file_no=1 |
Summary: | The Bantoum area in west Cameroon is composed of migmatitic gneisses associated with parallel strips of amphibolites,quartz-monzonites,biotite-granites, two-mica leucogranites and granitic dikes.Quartz-monzonites are metaluminous (A/CNK=0.8-0.9)I-type,biotite-granites are peraluminous (A/CNK=1.0-1.10)I-type, leucogranites are peraluminous (A/CNK=1.14)S-type granitoids.All are hyper-potassic rocks defining a calc-alkaline trend.Quartz-monzonites gave an Rb-Sr isochron age of 720+-61 Ma assumed to be a mixing age.The thermometry estimated from major elements and zircon saturation indicate that the biotite-granites crystallized from high temperature melts (812-866゜C) whereas leucogranites crystallized from low temperature melts (719-745゜C). The trace element distribution diagrams are characterized by an enrichment in LILE and LREE (5<La_N/Sm_N<17),with negative Nb,Ta,Sr and Ti anomalies. Model initial ^87Sr/^86 Sr ratios (620 Ma)are 0.707614-0.708363 for quartz-monzonites,0.711242-0.713784 for biotite-granites,and 0.715835 for leucogranites.They have highly negative ε_Nd (620 Ma)(-19~-11) and T_DM model ages ranging from 1.9 to 2.9 Ga. These geochemical and isotopic features imply that the granites are generated at different temperatures and from different crustal materials;they are the witnesses of the recycling of a Paleoproterozoic to Archean crust with minor inputs of juvenile magmas during the Pan-African orogeny. Chemical similarities between gneisses and some biotite-granites suggest that the partial melting of these gneisses may have contributed to the formation of granites. |
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