Ultramafic Rocks|| |
A. Def: ultramafic lavas
-Known only from Precambrian: composition: high MgO (18-33%); first described from Komati River, South Africa - also found in Canada, Australia, Finland
-Part of greenstone belts = arcuate zones of ancient metamorphosed sedimentary + igneous rocks
B. Typical flow consists of several layers:
-Top is highly fractured
-Next is spinifex zone = texture consisting of long criss-crossing blades of olivine crystals that radiate or fan out downward into flow
-Sharp boundary with fine- to medium-grained peridotite, which develops knobby weathering in lowest part
C. Experimental work
-Komatiite liquidus >1650oC => quench crystals likely to form
-High T also => magms likely caused partial melting and assimilation on the way up
-As a result, komatiites have highly variable concentrations of trace elements (contamination)
D. Archean geotherm
-High temperature of komatiites => geotherm was higher/steeper in Archean: today mantle melting generates basalts, would need >50% partial melting to get high Mg of komatiites; experimental work shows melting at high T & P (i.e, deeper) can produce magnesian melts with lower % melting; also, most igneous rocks in greenstone belts are basalts
-Conclusion: basaltic melts formed <100 km depth, komatiitic melts at 150-200km
-Aftter the Archean, geotherm intersected solidus at depths too shallow to make the latter
A. Def: unusual alkaline rock, often containing diamonds
-Found in cratons
-Ages = Precambrian - Cenozoic => stable source for these magmas.
-Has SiO2 = 30% and MgO = 23%
-Unlike other ultramafics, these have high TiO2, K2O, P2O5 & other incompatibles => derived by low % melting of fertile mantle
-Also, they have high Ni & Cr => no significant fractionation of olivine during ascent
C. Presence of diamonds + xenoliths suggest:
D. Occur in diatremes = pipelike body of breccia composed of fragments of country rock and mantle xenoliths, surrounded by kimberlite
-Diatremes are believed to be feeders to small shallow volcanic craters (maars) which form from gas explosions with little or no lava.
-Grades downward into massive dikes.
-"The emplacement of a diatreme resembles a cleanly drilled hole connecting the top of a dike or intersecting dike set with the surface of the Earth, the drilling products being left to fill the hole."