Petrogenetic evolution of the Koziakas ophiolite complex (W. Thessaly, Greece)

Summary The Koziakas ophiolitic complex is situated in W. Thessaly and is interpreted as an incomplete ophiolite sequence, thrust over the Western Thessaly Unit (W.T.U.). Two tectonically distinct units represent it: (1) the lower unit with a metamorphic sole and a tectonosedimentary mélange and (2) the upper unit including mantle peridotites and basaltic lavas. The mantle peridotites are composed by harzburgite, lherzolite and plagioclase lherzolite intruded by a sparse network of gabbroic, plagiogranitic and doleritic dykes. The volcanic sequence of the upper unit can be geochemically subdivided into four groups of basalts with: (1) tholeiitic N-type MORB affinities, (2) low-Ti boninitic affinities, (3) subalkalic E-MORB type affinities and (4) alkali characteristics displaying a different petrogenetic evolution with respect to the other groups. The magmatic history of the Koziakas ophiolite is in agreement with extensive fractional crystallization and variable degrees of partial melting of a heterogeneous mantle source, yielding, magmas mainly of MORB composition. Modal and cryptic metasomatic phenomena of the mantle peridotites as well as Sr-Nd isotopic ratios imply that this melt contained also a hydrous component derived from melting of a subducted lithosphere. The above geochemical characteristics and the correlation with the adjacent ophiolite suites of Pindos, Othris and Vourinos indicate that Koziakas ophiolitic complex formed in a small backarc basin situated at the eastern margin of the greater Pindos Ocean, between the Western Thessaly Unit (W.T.U.) and the Pelagonian continent.     

Quantitative modeling of Sr,Ca, Rb,and K in the Bjerkrem-Sogndal layered lopolith (S.W. Norway)

A systematic approach with graphic techniques is used to establish a quantitative model of fractional crystallization process in igneous layered complexes. Modeling of the evolution of Sr-Ca in plagioclase and K-Rb in plagioclase and whole rock coming from the Bjerkrem-Sogndal layered lopolith (Rogaland-S.W. Norway) is taken as an example. The relationships in logarithmic coordinates can be decomposed in a succession of segments. This permits identifying the Rayleigh law as controlling the process. A step by step solution is used to determine the parameters of the model which lead to the adjustment of the calculated evolution to the observed trend. Evidence in favour of an open system crystallization of the cumulate rocks permits determining the equilibrium partition coefficients between the various minerals and the liquid. The mean cumulate corresponding to a phase of crystallization of the intrusion is determined by averaging the mineral composition of the rocks belonging to that phase. The concentrations of the major elements Ca and K are used instead of activities. The adopted plagioclase-magma partition coefficients are close to those measured in anorthositic rocks for the same range of plagioclase composition between megacrysts and liquid. For an anorthite content of respectively 50, 43 and 31, D _Sr ^plag is equal to 2.0, 2.3 and 3.9, D _K ^plag varies between 0.40 and 0.25, D _Rb ^plag is either constant (ca. 0.10) or increases from 0.12 to 0.25, D _Ca ^plag is supposed to remain at an approximately constant value of 1.48. The fraction of residual liquid in the intrusion is 0.47 at the end of the anorthositic-leuconoritic phase, and 0.21 at the end of the cpxnoritic phase.     

CORUNDUM OF PING SHAN DISTRICT, W. CHIHLI.

I. GEOLOGY AND TOPOGRAPHY OF THE CORUNDUM DISTRTOT. The corundum is found in Ping Shan Hsien (平山縣) which is located in the south-western part of Chihli. It connects with Chin Hsing     

CORUNDUM OF PING SHAN DISTRICT, W. CHIHLI.

I. GEOLOGY AND TOPOGRAPHY OF THE CORUNDUM DISTRTOT. The corundum is found in Ping Shan Hsien (平山縣) which is located in the south-western part of Chihli. It connects with Chin Hsing     

Paleosols and the forest border in Keewatin,N.W.T.

The morphology of paleosols and radiocarbon-dated charcoal from buried surface horizons of soils provide evidence to suggest that between periods of northward forest encroachment tundra climate has dominated areas at least 50 km south of the present forest/tundra border in southwest Keewatin. The present forest/tundra border climate is nearly as severe as any climate that has prevailed in the area since deglaciation.     

Geomorphology and tectonics of the Dorrigo Plateau,N.S.W.

The Dorrigo Plateau is covered by basalt, which is a remnant of the 18 Ma old Ebor Volcano. The centre of this volcano is an intrusion in the Bellinger Valley. The volcano was erupted on a palaeoplain of moderate relief. Subsequent uplift and tilting led to erosion of the Nambucca Beds, together with much of the volcano, and creation of a major escarpment, part of the Great Escarpment of eastern Australia. In this area the Great Escarpment is younger than 18 Ma.