Petrology and geochemistry of syenite intrusions in the Eastern Province of Zambia
Date
2012-07-25
Authors
Tembo, Francis
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Abstract
Syenitic rocks in Eastern Zambia are represented by both oversaturated quartz bearing types and undersaturated nepholine-sodalite varieties.The ,Sinda syenite intrusions are oversaturated potassics which form a petrogenetically related N-S trending string of intrusions at the eastern margin of the Sinda batholith.The northernmost Lusandwa intrusion is a composite diapir covering some 70sq Km whilst the smaller Miyira, Seya, Pule and Tantha intrusions to the south are increasingly sill-like reflecting a strong structural control to their emplacement.-Petrographically, the syenites range from mafic pyroxene syenite through hornblende syenites to lencosyenites in the Lusandwa and Mayira intrusions,whilst the Pule, Seya and Tantha intrusions are homogeneous hornblende syenites.
The intrusions show systematic variations in their chemistry from the mafic syenites to the folsic syenites which are attributed to fractional crystallization of a mafic alkaline syenite magma whilst the Pule, Tantha and Seya intrusion deviate from this Mayira intrusions define an alkaline trend representing fractional crystallization of a mafic alkaline syenite magma v;hilst the Pule, Tantha and Seya intrusions deviate from this trend towards calc-alkaline compositions reflecting a change in the crystallizing assemblage induced hy changes in the pliysical conditions during crystallization.The apatite rich bodies within the Mayira intrusion, which are under investigation as phosphate ore have a mineralogy and mineral chemistry whicli indicates an origin as cumulates, v/ith subsequent enijlaccment at a late stage of solidification.The Lunkhwakwa intrusion in the Chipata area is an over-saturated potassic syenite which ranges in petrography from a mafic pyroxene syenite to a leucosyenite whilst the Bandawe intrusion in the south comprises a homogeneous sodi-potassic nepheline syenite.
The two intrusions also exhibit systematic chemical variations which are attributed to fractional crystallization.The chemistry of the Randawe syenite is significantly different from that of the Lunkhwakwa syenite suggesting that the two intrusions originated from two different magmas. The Bandawe magma is considered to be a derivative of an alkaline magma of mantle origin whilst the Lunkhwakwa magma has a partly crustal origin.The Mivula Hill intrusion is an undersaturated layered
intrusion which petrographically is a nepheline sodalite syenite.Chemically, the intrusion is miaskitic and peralkaline. The chemical variation within the exposed part of the intrusion is attributed to both fractional crystallization and crystal-liquid mixing processes with the latter predominating.Rb-?r isotope data have yielded an isochron with an age of 1340+16 Ma and an initial Sr isotopic ratio of 0.7028+0.0001. The Sr ratio suggests a mantle origin for the magma whilst the age is somewhat controversial and suggests the intrusion to have taken place in the early stages of the Kibaran orogeny.
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Keywords
Geochemistry -- Zambia -- Eastern Province , Petrology -- Zambia