Yu. G. Gatinskii and D. V. Rundkvist
Vernadskii National Geological Museum, Russian Academy of Sciences
The cycle of the formation and destruction of a supercontinent was suggested by J. Wilson in the early 1960s and was later named after him. This cycle includes the following successive stages: the convergence of continental masses with the closure of the oceanic basins separating them as the result of the predominance of the convergence and subduction of the lithospheric plates; their convergence into a large supercontinent as a result of the processes of collision, folding, and granitization; the breaking of the newly formed continental mass during a rifting activity with its breakup into individual continental blocks; the divergence of these blocks during the spreading activity and the formation of new oceanic basins. These cycles were repeated many times during the Proterozoic and Phanerozoic history of the Earth and are believed to have been associated with the transformation of single-cell mantle convection to multicell convection [Khain, 1995, 2001; Sorokhtin and Ushakov, 1993]. The last cycle began 250-300 million years ago and is still going on at the present time.
The Wilson cycles were accompanied by the alternation of oppositely directed geodynamic activities in the lithosphere. First of all, these were the replacements of the predominant compression and growth of the continental crust during the subduction and collision by its extension and disintegration as a result of rifting and spreading. Worthy of mention are also the transformations of extensive passive continental margins into active ones, the transformations of volcanic island arcs into continental-margin igneous rock belts, etc. These processes modified significantly the distribution of various metallogenic provinces and changed many of their characteristics [Gatinskii, 1985; Khain, 2000; Kovalev, 1985; Mitchell and Garson, 1981; Rundkvist, 1995]. Within the problem discussed in this number of the journal, it is of interest to trace a relationship between the various cycles of the Wilson cycle and the related mineral deposits with mantle plumes. But first, it is worthwhile discussing the modern ideas on the mantle plumes, their classifications in terms of size and depth, and the spatial association of various mineral deposits, combined into metallogenic belts or oil and gas basins, with the projections of plumes to the Earth's surface.
One of the most important achievements in the Earth sciences during the last decade was the discovery with the help of seismic tomography of large heat and density heterogeneities at various depths in the mantle and lithosphere. Their location was based first of all on the location of the regions of the slowdown and acceleration of seismic waves at various depth levels corresponding, according to the most widespread interpretation, to the hot ascending and cold descending flows of the material. The former are known as hot mantle plumes [Dziewonski and Woodhouse, 1987; Kumazawa and Maruyama, 1994]. It is significant that hot linear zones dominate at the most shallow depths ( < 200-350 km) under most of the active margins and continental rifts, whereas local isometric hot anomalies are observed at greater depths. Most of them concentrate in the central part of the Pacific Ocean and Africa and in the northwestern part of the Indian Ocean. Another important feature of the mantle plumes is the development of specific magmatism in the upper lithosphere above them [Grachev, 2000, 2002], represented by layered basic and ultrabasic intrusions, granites of high alkalinity and alkaline intrusive rocks, tholeiitic, subalkalic, and alkalic basaltoids, and contrasting volcanic rocks.
Relatively shallow ( < 600 km) hot plumes have been recorded also outside of the projections of the above mentioned large plums: under West Europe and Iceland, under Arabia and Near East, under Southeast Asia, and elsewhere. These plumes are usually interpreted as the far extending tongues of the superplumes. The most persistent downgoing cold flows, referred to as "cold plumes" by some researchers [Kumazawa and Maruyama, 1994], correspond primarily to the regions of the long-lasting subduction of the oceanic lithosphere under the continental plates.
Whereas the European and African magmatic areas were located within or near the Indian-African Superplume and the lines of the future Pangea breakup, the Siberian areas were notably remote from the nearest Greenland Plume. This is a surprising fact, because almost all of the subsequent trap lava flows were controlled undoubtedly by hot plumes and preceded the large breakups of the lithosphere in the regions of their occurrence. It appears that the enigma of the Siberian traps can be explained by the facts that our data for the Siberian tomography are still not complete, or that the paleoreconstructions available are not exact and that for the Early Triassic Siberia should be turned counterclockwise for a longer distance to be combined with the northern plume. It appears that later the rapid clockwise rotation of Siberia interrupted the beginning phase of this continent breakup. In any case, it is hard to agree with the view that Siberian traps flowed in the environment of dominating compression [Migurskii, 2002]. Most of the stratiform Pb-Zn and low-temperature Hg-Sb mineral deposits at the Pangea passive margins in North Africa, South-East Europe, and Arabia [Mitchell and Garson, 1981] were also controlled by the boundary of the Indian-African plume. Active continental margins with rare-metal and copper porphyry mineral deposits are known to have existed outside of the superplume projections in East Asia and East Australia (Figure 3).
During the Late Cenozoic the geodynamic conditions and mineralization types changed drastically on both sides of the Pacific (Figure 10). In the east the mid-oceanic ridge subsided partially under the continental plate. This caused the predominance of extension in the Basin and Range Province. Tin and rare earth ore deposits are associated with alkaline intrusions there (Mountain Pass and other ore deposits). Some other parts of the American active margin show the development of rare-metal and porphyry copper provinces above the areas of the subsidence of the young and hot lithosphere of the Pacific, Cocos, and Nasca plates. At the same time pyrites deposits dominate in the west in island arcs, where the older and cold lithosphere is subsiding, some of them being replaced along the strikes of the structures by porphyry copper deposits in the segments with the subduction of the hotter and lighter lithosphere [Gatinskii et al., 2000].
Extensive collision belts with mainly rare-metal provinces were formed in South Eurasia in the Alps, Himalayas, and in other regions [Mitchell and Garson, 1981]. The continental blocks were gradually combined around Eurasia which, possibly, will become the center of the formation of a new supercontinent, the origin of which will mark the end of the next Wilson Cycle.
In this paper we attempted to demonstrate that during the latest Paleozoic, Mesozoic, and Cenozoic the formation of the metallogenic provinces had been controlled by changes in the geodynamic conditions and by their positions relative to the projections of major hot plumes. The Wilson Cycle included the formation of Pangea in the environment dominated by the processes of subduction and collision, its disintegration above a deep superplume, dominated by rifting and spreading processes, and the subsequent combination of the continental blocks into a new supercontinent. This scenario seems to valid for the Precambrian and Early Phanerozoic history of the Earth, beginning from the transition of the chaotic multiple convection during the Hadean to the formation of individual continental nuclea at the beginning of the Archeozoic and of the first supercontinent during its end [Khain, 1995, 2001; Kumazawa and Maruyama, 1994; Sorokhtin and Ushakov, 1993]. This cycle was accompanied by a successive change of metallogenic provinces from primarily rare-metal ones via low-temperature hydrothermal, rare-metal and rare-earth, telethermal stratiform, pyrites, and Cu-Mo porphyritic provinces to new rare-metal ones. It appears that the formation of large oil and gas accumulations was restricted mainly to the early and intermediate stages of the cycle.
Two problems are offered here as the subjects of discussion. The first of them concerns the open and closed systems of plume tectonics operation in the upper layers of the lithosphere. The former are known to have developed in highly folded and faulted Precambrian massifs and Phanerozoic fold regions and are usually accompanied by metallic mineralization. Examples can be found in Africa, in Variscan Europe, and elsewhere.
The closed systems originated under large sedimentary basins which precluded the free rise of heat and fluids to the Earth surface, these fluids acting as sources of oil and gas pools (Persian Gulf, Western Siberia, etc.). The second problem concerns the zonal distribution of metallogenic provinces above the hot plumes. In many cases, though obviously not always, alkaline-granite and basic-ultrabasic magmatism and associated mineralization are restricted to the apical parts of the plumes, varying in depth, the provinces of telethermal and low-temperature mineralization, to the outer parts of the plumes, and the oil and gas basins, to the peripheral regions of the plume effects. However, these problems are beyond the framework of this paper.
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