Sino-Korean Craton (SKC) in eastern China is an important natural laboratory for studying temporal change to the lithosphere because there is the jutaxposition of Ordovician diamondiferous kimberlites, Mesozoic lamprophyre-basalt and Cenozoic tholeiite-alkali basalts in this craton. While diamond inclusions, xenoliths and mineral concentrates in kimberlites indicate a thick (180 km), cold and refractory lithospheric keel beneath the SKC prior to the Palaeozoic, basalt-borne xenoliths reveal the presence of thin (

Thermo-tectonic destruction of the archaean lithospheric keel beneath the sino-korean craton in china: evidence, timing and mechanism

The composition of the subcontinental lithospheric mantle (SCLM) is broadly related to the tectonothermal age of the overlying crust, suggesting a secular change in SCLM-forming processes. Most estimated compositions of Archean SCLM, based on well-studied suites of xenoliths and xenocrysts, are depleted garnet lherzolites with high orthopyroxene/olivine. However, these compositions make it difficult to account for the high shear-wave velocities measured in the cores of large cratons, and predict deeper geoid anomalies and higher elevations than are observed in most cratons. Global and regional seismic tomography indicates that most cratonic xenolith suites represent material from the lower-velocity margins of lithospheric blocks. This implies that previous compositional estimates are strongly biased toward metasomatized material. We suggest that most Archean SCLM originally consisted of highly depleted dunites/ harzburgites, similar to the Archean orogenic massifs of western Norway. Incorporation of such rocks in the cold upper parts of the cratonic SCLM satisfies the seismic and gravity data, suggesting that large volumes of these rocks are preserved in the cores of cratons, but are poorly sampled by volcanic rocks. The roots of most Proterozoic shields probably consist of refertilized Archean SCLM; the juvenile SCLM beneath Proterozoic and Phanerozoic mobile belts reflects only moderate depletion of Primitive Mantle compositions. Rather than a gradual evolution in SCLM-forming processes, we suggest a sharp dichotomy between Archean and younger tectonic regimes.The differences in buoyancy and viscosity between these two types of SCLM have played a major role in the construction, preservation and recycling of continental crust. If originally Archean SCLM is more widespread than currently recognized, models of crustal growth rates and recycling may need to be revised.

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The Composition and Evolution of Lithospheric Mantle: a Re-evaluation and its Tectonic Implications

Orogenic, ophiolitic, and abyssal peridotites represent subcontinental, suboceanic, and subarc mantle rocks that were exhumed to the surface in various tectonic settings. These rocks provide coverage of vast regions of the Earth's upper mantle that are sparsely sampled by mantle xenoliths. They notably allow the observation of a wide range of lithospheric mantle compositions, including cratonic roots and subduction mantle wedges (high-pressure orogenic garnet lherzolites), variably rejuvenated subcontinental lithosphere (lower-pressure orogenic spinel and plagioclase lherzolites), and newly accreted oceanic lithosphere (ophiolitic mantle and abyssal peridotites). It is shown here that most of geochemical variability recorded by these mantle rocks is attributable to melt processes associated with partial melting and asthenosphere–lithosphere interactions. Rather than remnants of pristine mantle, the fertile orogenic lherzolites are now widely considered as former refractory lithospheric mantle refertilized by upwelling partial melt.

Orogenic, ophiolitic and abyssal peridotites

http://www.gm.univ-montp2.fr/PERSO/tommasi/LeRoux-etal-EPSL2007.pdf

The Lherz spinel lherzolite: Refertilized rather than pristine mantle

Mineral and whole-rock chemical data for peridotite xenoliths in basaltic lavas on Spitsbergen are examined to reassess mechanisms of melt-fluid interaction with peridotites and their relative role versus melt composition in mantle metasomatism. The enrichment patterns in the xenoliths on primitive mantle-normalized diagrams range from Th-La-Ce 'inflections' in weakly metasomatized samples (normally without amphibole) to a continuous increase in abundances from Ho to Ce typical for amphibole-bearing xenoliths. Numerical modelling of interaction between depleted peridotites and enriched melts indicates that these patterns do not result from simple mixing of the two end-members but can be explained by chromatographic fractionation during reactive porous melt flow, which produces a variety of enrichment patterns in a single event. Many metasomatized xenoliths have negative high field strength element and Pb anomalies and Sr spikes relative to rare earth elements of similar compatibility, and highly fractionated Nb/Ta and Zr/Hf. Although amphibole precipitation can produce Nb-Ta anomalies, some of these features cannot be attributed to percolation-related fractionation alone and have to be a signature of the initial melt (possibly carbonate rich). In general, chemical and mineralogical fingerprints of a metasomatic medium are strongest near its source (e.g. a vein) whereas element patterns farther in the metasomatic 'column' are increasingly controlled by fractionation mechanisms.

Mechanisms and Sources of Mantle Metasomatism: Major and Trace Element Compositions of Peridotite Xenoliths from Spitsbergen in the Context of Numerical Modelling

It is well established that porous melt flow in the upper mantle may significantly affect partial mantle melt compositions. Less well established are the length-scale of porous flow and whether porous melt flow can be a volumetrically important magmatic process. The only source for observations concerning the length-scale and nature of pervasive porous melt flow are peridotite massifs. Here we present such observations in the form of structural, and major and trace element data from peridotites of the Ronda massif, southern Spain. Trace element concentrations were obtained with high analytical precision (ICP-MS) and include trace elements rarely analysed in peridotites, such as Rb, Th, Nb and Ta. The western portion of the Ronda massif can be divided into two structural facies. The first and oldest is composed of deformed, porphyroclastic spinel peridotites, the second of virtually undeformed granular spinel peridotites. They are separated by a recrystallisation front across which grain growth of all phases occurred. The granular domain can be further subdivided into three subfacies: coarse-granular, fine-granular, and layered-granular peridotites. According to structural facies, km-scale spatial variations unrelated to Ca and Al abundances have been recognised for mg-numbers [atomic Mg/(Mg±Fe)] and incompatible elements such as rare earth elements (REE), Th and high-field-strength elements (HFSE; including Ti). Such variations are reminiscent of those commonly ascribed to mantle metasomatism, but have never been documented on the km-scale. The origin of the recrystallisation front is related to km-scale pervasive melt percolation. Feed-back processes between grain growth and melt fraction could have led to important accumulation of melt at the recrystallisation front, accomplished mainly by melting/dissolution. Variation in melt fraction across the front explains the spatial variation in the degree of recrystallisation, mg-numbers, REE fractionation, and HFSE abundances, and could account for many of the classical differences between basalts from convergent and extensional tectonic settings. Whereas the coarse-granular peridotites reflect a stage of steady-state pervasive porous melt flow, the fine- and layered-granular facies probably reflect the terminate stages of porous melt flow. Processes associated with both domains are pyroxene-forming freezing reactions at decreasing melt volumes, and progressive channelling of melt flow associated with olivine-producing reactions. Both processes show complex overprinting relationships in both time and space.

D. Van der Wal

Papers

Origin of the recrystallisation front in the Ronda peridotite by km-scale pervasive porous melt flow