Postęp w badaniach ofiolitów
Abstract
DEVELOPMENTS IN OPHIOLITE STUDIES
Summary
The definition of petrologic-structural term "ophiolite" in its present usage was accepted by participants of the Ophiolite Conference in Penrose, 1972. According to the definition, ophiolite means a close assemblage of ultramafic and mafic igneous rocks with a peculiar composition and a specific vertical sequence. The Troodos Massif, Cyprus (fig. 2) is considered to be the standard ophiolite complex, as it is fully developed, well exposed, underformed and most extensively studied ophiolitic suite around the world. The Troodos ophiolite complex consists of the three following parts listed from top to bottom:
l) extrusive series of pillow lavas: olivine basalts, tholeiites, picrites with minor limburgites and andesites
2) intrusive series of diabase sheeted dikes
3) plutonic complex consisting of high level intrusives: gabbroic to plagiogranitic rocks layered ultrabasic and basic cumulates: peridotites, troctolites and other gabbros, which overly tectonized harzburgites representing the depleted uppermost Earths mantle.
The sequence of the plutonic complex can be accounted for on the basis of crystallization with the steady-state magma chamber with a permanent supply, i.e. the magma reservoir existing beneath a spreading axis of an oceanic ridge (fig. 3). Crystallization takes place there under dynamic conditions.
Geochemical investigations of ophiolitic suites allow to estimate the chemical composition of a primary material for ophiolites, to trace a sequence and intensity of magmatic processes as well as to reconstruct the past geological setting of the upper parts of ophiolites (figs. 5, 6 and 7).
During the lnternational Ophiolite Symposium in Nicosia, Cyprus in April 1979 ophiolites were viewed upon as remnants of ancient oceanic lithosphere incorporated into orogenic structures. All the problems of ophiolites were considered on the basis of plate tectonics. An influence of a plate descending in process of subduction on the oceanization (basification) of and overlying crust was also discussed (fig. 8).
The Troodos Massif has originated in a marginal back-arc basin of the Tethys, above a palaeo-subduction zone of the African plate. The uplift of the massif occurred at the turn of the Cretaceous and Tertiary.
It is nowadays believed that the majority of ophiolites around the world originated within the Western Pacific-type marginal seas, i.e. within small basins bordered by island arcs on one side and continental margins on the other.
During the Ophiolites Symposium there were also presented and discussed results of investigations of present-day oceanic lithosphere and its comparison with ophiolite sequence.
Summary
The definition of petrologic-structural term "ophiolite" in its present usage was accepted by participants of the Ophiolite Conference in Penrose, 1972. According to the definition, ophiolite means a close assemblage of ultramafic and mafic igneous rocks with a peculiar composition and a specific vertical sequence. The Troodos Massif, Cyprus (fig. 2) is considered to be the standard ophiolite complex, as it is fully developed, well exposed, underformed and most extensively studied ophiolitic suite around the world. The Troodos ophiolite complex consists of the three following parts listed from top to bottom:
l) extrusive series of pillow lavas: olivine basalts, tholeiites, picrites with minor limburgites and andesites
2) intrusive series of diabase sheeted dikes
3) plutonic complex consisting of high level intrusives: gabbroic to plagiogranitic rocks layered ultrabasic and basic cumulates: peridotites, troctolites and other gabbros, which overly tectonized harzburgites representing the depleted uppermost Earths mantle.
The sequence of the plutonic complex can be accounted for on the basis of crystallization with the steady-state magma chamber with a permanent supply, i.e. the magma reservoir existing beneath a spreading axis of an oceanic ridge (fig. 3). Crystallization takes place there under dynamic conditions.
Geochemical investigations of ophiolitic suites allow to estimate the chemical composition of a primary material for ophiolites, to trace a sequence and intensity of magmatic processes as well as to reconstruct the past geological setting of the upper parts of ophiolites (figs. 5, 6 and 7).
During the lnternational Ophiolite Symposium in Nicosia, Cyprus in April 1979 ophiolites were viewed upon as remnants of ancient oceanic lithosphere incorporated into orogenic structures. All the problems of ophiolites were considered on the basis of plate tectonics. An influence of a plate descending in process of subduction on the oceanization (basification) of and overlying crust was also discussed (fig. 8).
The Troodos Massif has originated in a marginal back-arc basin of the Tethys, above a palaeo-subduction zone of the African plate. The uplift of the massif occurred at the turn of the Cretaceous and Tertiary.
It is nowadays believed that the majority of ophiolites around the world originated within the Western Pacific-type marginal seas, i.e. within small basins bordered by island arcs on one side and continental margins on the other.
During the Ophiolites Symposium there were also presented and discussed results of investigations of present-day oceanic lithosphere and its comparison with ophiolite sequence.