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A m ed iu m g rained ro ck o f porp h y ro b lastic tex tu re d efined by coarser c linopyroxene, o liv in e and dark m ica, w ith in te rg ra n u la r d o m ains o f fine-grained, seco n d -g en eratio n olivine, clin opyroxene and d a rk m ica, to g eth e r w ith a brow n glass. T he large p o rp h yroblasts are m ain ly c lin o p y ro x en e w ith subordinate olivine and m ica. T he po rp h y ro b lastic olivine show s rare co arse strain lam ellae and m ica has closely spaced, fine strain lam ellae. T he clin o p y ro x en e porp h y ro b lasts are fractured, h ave irregular b o u n d aries and un d u lo se ex tin ctio n (Fig. 5.2 d-e). M o st o f th em are z o n ed (Fig. 5.2 f). In a few cases larger clin o p y ro x en e crystals along w ith com p o sitio n al zo n in g h ave ex so lu tio n lam ellae, and th ese m ay be d isplaced (Fig. 5.2 f) suggesting th at d efo rm atio n fo llow ed th eir form ation. T here is also som e brecciatio n to g eth er w ith dev elo p m en t o f n eo blasts along clin o p y ro x en e and olivine p o rp h y ro b last grain boundaries. A n u m b er o f th e sm a lle r porp h y ro b lasts have

dev elo p ed su b hedral faces and rim ov erg ro w th s ag ain st the interstitial

pyro m etam o rp h ic assem blage. T rip le ju n c tio n s and sharp p lan a r o r cu rv ilin ear contacts are so m etim es preserved.

The m etasom atised m antle beneath Mt. Vulture volcano

C lin o p y ro x en e is typically pale b row n and w eak ly pleochroic. T he p o rp h y ro b la st cores show a range o f features th a t d o c u m e n t th e progressive rep lacem en t o f orth o p y ro x en e by clinopyroxene:

(1) C ores o f rounded orthopyroxene relicts w ith a reactio n co rona o f olivine (Fig. 5.2 a-b), in w hich m ay be p resen t sm all g lassy patches.

(2) C ores w ith an irregular-shaped aggregate o f o liv in e grains th at m ay trap interstitial glass (Fig. 5.2 b).

(3) C ore w ith only trapped glass in clusions (Fig. 5.2 d).

(4) M ore co m m o n ly sub -ro u n d ed co lo u rless core in apparent

cry stallographic/optical c o n tin u ity w ith the enclosing brow n

clinopyroxene.

(5) R arely, a sharply defined core o f c o lo u rless clin o p y ro x en e is com posed o f a sub-grain m osaic.

(6) In one case the clin opyroxene core is o f m ica surrounded by an intergrow th o f euhedral olivine grains (Fig. 5.2 e).

O ccasio n ally ro u n d ed d ark b row n spinel grains are asso ciated w ith porp h y ro b lastic o livines and clinopyroxenes. S pinel g rains frequently show o v ergrow ths, and som e o f them h av e a sponge tex tu re w ith sm all clinopyroxene inclusions. T riple ju n c tio n s b etw een the spinel and the su rro u n d in g clinopyroxene suggest a p o st-p artial m eltin g com plete an n ealin g and requ ilib ratio n .

G lass is d ark b ro w n and contains m icro p h en o cry sts o f clinopyroxene, apatite, b leb s o f carbonate (10-20 p m in diam eter) and p h lo g o p ite relicts (C h ap ter 6. par. 6.4.3).

The m etasom atised mantle beneath Mt. Vulture volcano

100 urn 100 M-m

F igu re 5.2. a) PPL (m icroscope plane polarised light) photo o f clinopyroxene (cpx) porphyroblast having orthopyroxene (opx) core surrounded by olivine (ol) reaction corona, b) B SE (Back-scattered Secondary Electron) im age o f the olivine reaction corona: the olivine crystals grew perpendicularly to the resorbed orthopyroxene margin, c) PPL photo show ing porphyroblastic clinopyroxene containing olivine crystal aggregate with trapped dark glass patches (gl). d) CPL (m icroscope cross-polarised light) photo o f clinopyroxene containing glass and CO] inclusions. The clinopyroxene is surrounded by phlogopite (phi) and reddish glass, e) CPL photo o f clinopyroxene containing olivine crystal aggregate enclosing phlogopite. N ote the clinopyroxene undulose extinction, f) CPL photo o f clinopyroxene porphyroblast with exsolution lam ellae and com positional zoning. The exsolution lam ellae may be displaced (red arrow).

The m etasom atised m antle beneath Mt. Vulture volcano