Mineralogical Compositions and Metamorphic Intensities of Contact Aureoles of Igneous Intrusive from the Lower Benue Trough, Nigeria

Abstract - Igneous intrusives of basic to intermediate (syenilic) compositions in the Lowcr

Bcnue Trough had produced contact cffccts which ro.lge from the lowcr albite- cpidotc

hornfels to the cordierite hornfels facies on the host sedimentary rocks which vary mainly from

pelitic to calcareous pelitic types. Contact effects of low intensity and size were produced by

three smaller igneous bodies of basic composition whose thicknesses range from 1 to 3 m. The

mineral assemblages of the aureole rocks include quartz, calcite, chlorite, and biotite; these

represent temperature conditions greater than 300°C, but not exceeding 450°C. the upper limit

or Uie albite-epidotc liorn~cls l'ilcies. l'hc rnaxinluni w~dlli ol' each aurcolc is 1.5 111.

Conversely, high intensity aureoles of large dimensions were produced by larger bodies of

intermediate compositions. One of these, an alkali dioritic rock of 2.0 km diameter, produced a

21 tn aureole characterized by a high temperature assemblage of calcite, enstatite and quartz

which represents a temperature condition grcaler than 600 "C. A ncpheline syenile which is

3.5 x 2.0 km in dimension produced a zoned aureole which extends up to a distance of 350 111

away from the contact with the host rock. 'The aureole contains cordierite, orthoclase, and

biotite in the innermost part, orthoclase and biotite in the middle and muscovite, chlorite and

quartz in the outermost part. These zones represent temperature conditions > ｿ 600 "C,