The Chinamora batholith, Zimbabwe: structure and emplacement-related magnetic rock fabric

The origin of dome-and-keel structural geometries in Archean granite-greens tone terrains appears to lack any modern analogues and is still poorly unde rstood. The formation of these geometries is investigated using structural and anisotropy of magnetic susceptibility (AMS) data for the Chinamora bath olith in Zimbabwe. The roughly circular-shaped batholith is surrounded by c a. 2.72-2.64 Ga greenstones. The batholith granitoid suites have been divid ed on the basis of their ages and fabric relationships into four distinct u nits: (i) banded basement gneisses; (ii) granodioritic gneisses; (iii) equi granular granites; and (iv) central porphyritic granites. In the gneissic g ranites a partial girdle (N-S) of poles to the magnetic foliation is develo ped that has been folded around a consistent, flat lying magnetic lineation plunging at shallow angles to the E or W. In the equigranular granites, th e magnetic lineation generally plunges to the NW. The magnetic foliation ha s a variable strike, no clear trends can be distinguished. The AMS measurem ents of the porphyritic granite revealed a NW-SE striking foliation and sho wed subhorizontal magnetic lineations. The magnetic foliation is subparalle l to the macroscopic foliation. Wall rocks are moderately inclined and show radial or concentric lineations, triaxial strain ellipsoids and kinematics that demonstrate off-the-dome sliding and coeval pluton expansion. The res ults of the observations do not point to a single emplacement process. Neit her the observed structural data nor the magnetic fabric support a model en visaging spherically 'ballooning'. It is argued that pluton diapirism playe d a major part in the formation of the fabrics in the gneisses, whereas the fabrics in the porphyritic granites reflect emplacement as laccolith-like sheets. (C) 2000 Elsevier Science Ltd. All rights reserved.