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The Ratama pluton—A specialised granite in the Midyan Region, Kingdom of Saudi Arabia
A35
THE R A T A M A PLUTON -- A SPECIALISED GRANITE IN THE MIDYAN REGION, KINGDOM OF SAUDI A R A B I A
C.R. RAMSAY and M.N. ROWAIHI
Directorate General of Mineral Resources, Jeddah (Saudi Arabia)
In the Midyan region, east of the Gulf of Aqaba, Late Proterozoic post-tectonic granodiorites, monzogranites, syenogranites, alkali feldspar granites, and arfvedsonite granites comprise a large proportion of the exposed rock. The roof zone of the Ratama pluton, at 29 ° 05 ~ N and 35 ° 20 wE, is exposed in a window (measuring ca. 5 × 3 kin) through lightly metamorphosed sediments and volcanics. The rock is homogeneous, equigranular, mediumto coarse-grained alkali feldspar granite. It is composed mainly of perthite and quartz, with 0--5% biotite, 0--2% muscovite and plagioclase, and accessory zircon and fluorite. Its general appearance is unexceptional and it has no anomalous radioactive signature. The major oxide composition is metaluminous and close to the average for Midyan granitoid rocks. The norm consists mainly of Ab (32--37%), Q (34--37%), and Or (25-26%), with very little An (1--3%). Neither corundum nor acmite are present, but there are significant amounts of magnetite (0--2%) and hematite (0--2%). In major-oxide variation diagrams the granite plots on or close to the main evolutionary trends of c o m m o n granites in the area, but has relatively high S i O J A I : O 3 and Fe:O3/FeO ratios, and low CaO, MgO and TiO 2. The trace-element suite is very distinctive, with the lithophile elements highly enriched. Be (5--10 ppm), F (1300--3000), Li (4--119), Nb (115--160) and Y (111--192) are anomalously high in most of the Ratama samples, while Zn (30--145), Sn (11--221), and Ta (0--28) are high in some samples. Th, U and Zr are slightly enriched but there is no unusual enrichment of Ce or La. There is strong depletion of Ba (ca 10 ppm) and probably Sr (less than 100 ppm). The Ratama granite therefore appears to be a plumasitic specialised granite which is a highly differentiated, lithophile-element enriched, variant of the c o m m o n I-type granites (if the Midyan region.
PETROGRAPHY, GEOCHEMISTRY A N D TECTONIC SETTING OF METAVOLCANIC SEQUENCES FROM THE AL-LITH AREA, SOUTHWESTERN A R A B I A N SHIELD
THOMAS REISCHMANN and A . K R O N E R
Mainz University, Mainz (W. Germany) A. BASAHEL
Faculty of Earth Sciences, King A bdulaziz University, Jeddah (Saudi Arabia)
This paper reports on seven Pan-African metavolcanic sequences in the southwestern Arabian shield that have been considered as equivalents of the Baish and Jiddah Groups by previous authors. These rocks display variable degrees of alteration ranging from lowtemperature autometamorphism to amphibolite-grade regional metamorphism. Petrographic
THE R A T A M A PLUTON -- A SPECIALISED GRANITE IN THE MIDYAN REGION, KINGDOM OF SAUDI A R A B I A
C.R. RAMSAY and M.N. ROWAIHI
Directorate General of Mineral Resources, Jeddah (Saudi Arabia)
In the Midyan region, east of the Gulf of Aqaba, Late Proterozoic post-tectonic granodiorites, monzogranites, syenogranites, alkali feldspar granites, and arfvedsonite granites comprise a large proportion of the exposed rock. The roof zone of the Ratama pluton, at 29 ° 05 ~ N and 35 ° 20 wE, is exposed in a window (measuring ca. 5 × 3 kin) through lightly metamorphosed sediments and volcanics. The rock is homogeneous, equigranular, mediumto coarse-grained alkali feldspar granite. It is composed mainly of perthite and quartz, with 0--5% biotite, 0--2% muscovite and plagioclase, and accessory zircon and fluorite. Its general appearance is unexceptional and it has no anomalous radioactive signature. The major oxide composition is metaluminous and close to the average for Midyan granitoid rocks. The norm consists mainly of Ab (32--37%), Q (34--37%), and Or (25-26%), with very little An (1--3%). Neither corundum nor acmite are present, but there are significant amounts of magnetite (0--2%) and hematite (0--2%). In major-oxide variation diagrams the granite plots on or close to the main evolutionary trends of c o m m o n granites in the area, but has relatively high S i O J A I : O 3 and Fe:O3/FeO ratios, and low CaO, MgO and TiO 2. The trace-element suite is very distinctive, with the lithophile elements highly enriched. Be (5--10 ppm), F (1300--3000), Li (4--119), Nb (115--160) and Y (111--192) are anomalously high in most of the Ratama samples, while Zn (30--145), Sn (11--221), and Ta (0--28) are high in some samples. Th, U and Zr are slightly enriched but there is no unusual enrichment of Ce or La. There is strong depletion of Ba (ca 10 ppm) and probably Sr (less than 100 ppm). The Ratama granite therefore appears to be a plumasitic specialised granite which is a highly differentiated, lithophile-element enriched, variant of the c o m m o n I-type granites (if the Midyan region.
PETROGRAPHY, GEOCHEMISTRY A N D TECTONIC SETTING OF METAVOLCANIC SEQUENCES FROM THE AL-LITH AREA, SOUTHWESTERN A R A B I A N SHIELD
THOMAS REISCHMANN and A . K R O N E R
Mainz University, Mainz (W. Germany) A. BASAHEL
Faculty of Earth Sciences, King A bdulaziz University, Jeddah (Saudi Arabia)
This paper reports on seven Pan-African metavolcanic sequences in the southwestern Arabian shield that have been considered as equivalents of the Baish and Jiddah Groups by previous authors. These rocks display variable degrees of alteration ranging from lowtemperature autometamorphism to amphibolite-grade regional metamorphism. Petrographic