Whole-rock geochemistry of basic and intermediate intrusive rocks in the Ishiagu area: further evidence of anorogenic setting of the Lower Benue rift, southeastern Nigeria

Petrographic studies on the intrusive rocks from the Ishiagu area show that they are gabbro, dolerites, and diorite. The gabbro and diorite occur as stocks while the dolerites occur as sills in folded sedimentary sequence predominated by shale. Geochemical data confirmed the rocks as basic (gabbro and dolerites) and intermediate (diorite) rocks with silica contents ranging from 42.45 to 50.75 wt.% and 55.03 to 60.24 wt.%, respectively. They are predominantly alkaline (gabbro, dolerites, and diorites) with sparse tholeiites (few gabbro). The basic rocks are made up of calcic plagioclase (An49-54), clinopyroxene, olivine (altered), nepheline, and opaques, while the intermediate rocks consist of plagioclase (An41-49), hornblende, and clinopyroxene. Calcite occurs as a secondary mineral in both groups. Although many samples are moderately to heavily altered (loss on ignition: 2.12–10.46 wt.%), a consistent decrease in magnesium number (Mg#) (gabbro, 0.47; dolerites, 0.37; dolerites, 0.27) and immobile elements (Y, Zr, Hf, Ti, Nb, and Ta), which are more enriched in the intermediate than the basic rocks and similar patterns of spidergrams and rare earth elements (REEs), show that the rocks (gabbro, dolerites, and diorites) are genetically related. The rocks show stronger enrichment in incompatible elements and REEs [(La/Yb)n, 4.83–8.15 for the gabbro, 7.01–7.79 for the dolerites, and 8.85–9.63 for the diorites]. Primitive-mantle normalized trace-element patterns and trace-element ratios of both basic and intermediate rocks indicate similarity to ocean island basalts, which suggests a garnet-lherzolite mantle source. The ratios of La/Nb, Th/La, and Th/Nb and the high ratio of Nb/La (1.18–1.50 for gabbro, 1.37–1.95 for dolerites, and 1.21–1.32 for diorites) indicate a high 238U/204Pb (HIMU) mantle source region. There is no evidence of significant crustal contamination on the basic and intermediate rocks, which may be related to the rapid ascent of the magmas in an intracontinental rift setting similar to the East African rift.

Whole-rock geochemistry of basic and intermediate intrusive rocks in the Ishiagu area: further evidence of anorogenic setting of the Lower Benue rift, southeastern Nigeria

Petrographic studies on the intrusive rocks from the Ishiagu area show that they are gabbro, dolerites, and diorite. The gabbro and diorite occur as stocks while the dolerites occur as sills in folded sedimentary sequence predominated by shale. Geochemical data confirmed the rocks as basic (gabbro and dolerites) and intermediate (diorite) rocks with silica contents ranging from 42.45 to 50.75 wt.% and 55.03 to 60.24 wt.%, respectively. They are predominantly alkaline (gabbro, dolerites, and diorites) with sparse tholeiites (few gabbro). The basic rocks are made up of calcic plagioclase (An49-54), clinopyroxene, olivine (altered), nepheline, and opaques, while the intermediate rocks consist of plagioclase (An41-49), hornblende, and clinopyroxene. Calcite occurs as a secondary mineral in both groups. Although many samples are moderately to heavily altered (loss on ignition: 2.12–10.46 wt.%), a consistent decrease in magnesium number (Mg#) (gabbro, 0.47; dolerites, 0.37; dolerites, 0.27) and immobile elements (Y, Zr, Hf, Ti, Nb, and Ta), which are more enriched in the intermediate than the basic rocks and similar patterns of spidergrams and rare earth elements (REEs), show that the rocks (gabbro, dolerites, and diorites) are genetically related. The rocks show stronger enrichment in incompatible elements and REEs [(La/Yb)n, 4.83–8.15 for the gabbro, 7.01–7.79 for the dolerites, and 8.85–9.63 for the diorites]. Primitive-mantle normalized trace-element patterns and trace-element ratios of both basic and intermediate rocks indicate similarity to ocean island basalts, which suggests a garnet-lherzolite mantle source. The ratios of La/Nb, Th/La, and Th/Nb and the high ratio of Nb/La (1.18–1.50 for gabbro, 1.37–1.95 for dolerites, and 1.21–1.32 for diorites) indicate a high 238U/204Pb (HIMU) mantle source region. There is no evidence of significant crustal contamination on the basic and intermediate rocks, which may be related to the rapid ascent of the magmas in an intracontinental rift setting similar to the East African rift.

Kaynakça

Abdel-Fattah M, Abdel-Rahman AM, Nassar PE (2004). Cenozoic volcanism in the Middle East: petrogenesis of alkali basalts from northern Lebanon. Geol Mag 141: 545–563.

Benkhelil J (1986). Structure and geodynamic evolution of the intracontinental Benue Trough (Nigeria). PhD, University of Nice, France.

Benkhelil J (1987). Cretaceous deformation, magmatism and

Kaynak Göster

Bibtex @ { tbtkearth143911, journal = {Turkish Journal of Earth Sciences}, issn = {1300-0985}, eissn = {1303-619X}, address = {}, publisher = {TÜBİTAK}, year = {2014}, volume = {23}, pages = {427 - 443}, doi = {10.3906/yer-1303-16}, title = {Whole-rock geochemistry of basic and intermediate intrusive rocks in the Ishiagu area: further evidence of anorogenic setting of the Lower Benue rift, southeastern Nigeria}, key = {cite}, author = {Chukwu, Anthony and Obıora, Smart Chika} }
APA Chukwu, A , Obıora, S . (2014). Whole-rock geochemistry of basic and intermediate intrusive rocks in the Ishiagu area: further evidence of anorogenic setting of the Lower Benue rift, southeastern Nigeria . Turkish Journal of Earth Sciences , 23 (4) , 427-443 . DOI: 10.3906/yer-1303-16
MLA Chukwu, A , Obıora, S . "Whole-rock geochemistry of basic and intermediate intrusive rocks in the Ishiagu area: further evidence of anorogenic setting of the Lower Benue rift, southeastern Nigeria" . Turkish Journal of Earth Sciences 23 (2014 ): 427-443 <https://dergipark.org.tr/tr/pub/tbtkearth/issue/12041/143911>
Chicago Chukwu, A , Obıora, S . "Whole-rock geochemistry of basic and intermediate intrusive rocks in the Ishiagu area: further evidence of anorogenic setting of the Lower Benue rift, southeastern Nigeria". Turkish Journal of Earth Sciences 23 (2014 ): 427-443
RIS TY - JOUR T1 - Whole-rock geochemistry of basic and intermediate intrusive rocks in the Ishiagu area: further evidence of anorogenic setting of the Lower Benue rift, southeastern Nigeria AU - Anthony Chukwu , Smart Chika Obıora Y1 - 2014 PY - 2014 N1 - doi: 10.3906/yer-1303-16 DO - 10.3906/yer-1303-16 T2 - Turkish Journal of Earth Sciences JF - Journal JO - JOR SP - 427 EP - 443 VL - 23 IS - 4 SN - 1300-0985-1303-619X M3 - doi: 10.3906/yer-1303-16 UR - https://doi.org/10.3906/yer-1303-16 Y2 - 2021 ER -
EndNote %0 Turkish Journal of Earth Sciences Whole-rock geochemistry of basic and intermediate intrusive rocks in the Ishiagu area: further evidence of anorogenic setting of the Lower Benue rift, southeastern Nigeria %A Anthony Chukwu , Smart Chika Obıora %T Whole-rock geochemistry of basic and intermediate intrusive rocks in the Ishiagu area: further evidence of anorogenic setting of the Lower Benue rift, southeastern Nigeria %D 2014 %J Turkish Journal of Earth Sciences %P 1300-0985-1303-619X %V 23 %N 4 %R doi: 10.3906/yer-1303-16 %U 10.3906/yer-1303-16
ISNAD Chukwu, Anthony , Obıora, Smart Chika . "Whole-rock geochemistry of basic and intermediate intrusive rocks in the Ishiagu area: further evidence of anorogenic setting of the Lower Benue rift, southeastern Nigeria". Turkish Journal of Earth Sciences 23 / 4 (Nisan 2014): 427-443 . https://doi.org/10.3906/yer-1303-16
AMA Chukwu A , Obıora S . Whole-rock geochemistry of basic and intermediate intrusive rocks in the Ishiagu area: further evidence of anorogenic setting of the Lower Benue rift, southeastern Nigeria. Turkish Journal of Earth Sciences. 2014; 23(4): 427-443.
Vancouver Chukwu A , Obıora S . Whole-rock geochemistry of basic and intermediate intrusive rocks in the Ishiagu area: further evidence of anorogenic setting of the Lower Benue rift, southeastern Nigeria. Turkish Journal of Earth Sciences. 2014; 23(4): 427-443.
IEEE A. Chukwu ve S. Obıora , "Whole-rock geochemistry of basic and intermediate intrusive rocks in the Ishiagu area: further evidence of anorogenic setting of the Lower Benue rift, southeastern Nigeria", Turkish Journal of Earth Sciences, c. 23, sayı. 4, ss. 427-443, Nis. 2014, doi:10.3906/yer-1303-16