Restoration of premetasomatic protolith compositions in mantle xenoliths
1
Citation
0
Reference
10
Related Paper
Citation Trend
Keywords:
Protolith
Xenolith
Protolith
Petrogenesis
Basement
Cite
Citations (16)
Cite
Citations (13)
Granitoids play a key role in the geological structure of the Ros-Tikych megablock. Supercrustal rocks of the Ros-Tikych series have been preserved in the granitoids only in the form of isolated fragments such as elongated remains, small skialites and even smaller "melted" xenoliths. In particular, in the Ostrivsky quarry, located on the right bank of the Ros River east of Bila Tserkva, granitoids are found (even-grained, porphyry-like granites) among which, as a rule, small bodies of granodiorites, plagiogranites and amphibolites occur. In order to determine the source of the parent magmas of rocks the properties of zircon crystals and the isotopic composition (87Sr/86Sr ratio) of apatite were studied. An analysis of the zircon crystals of the crystalline rocks exposed at the Ostrivsky quarry allows us to propose that the and plagio- and difeldspar granites were formed from one protolith. This is because they contain similar virtually identical zircon relics as nucleus. In addition, none of the granitoids contain zircon crystals whose internal structure is similar to zircon crystals found in amphibolite. This suggests that the granitoids were not derived by melting of amphibolites. Most likely, amphibolites are relicts of the protolith that were not assimilated during granite formation. The occurrence of heterogeneous zircon crystals (relic zircon cores of the protolith) in the protolith of the various studied granitoids indicates that they formed from volcanic-sedimentary rocks. Apatites in plagiogranitoids and porphyry granite contain strontium of similar isotopic composition. Their 87Sr/86Sr isotopic ratio is 0.70680 in apatite granodiorite and 0.70822 in granite. A high ratio of 87Sr/86Sr = 0.77940 was measured for apatite from monazite-bearing granite, thus indicating a different source for its parent magma.
Protolith
Cite
Citations (1)
Protolith
Felsic
Cite
Citations (22)
Abundant metabasites occur in highly deformed granitic and migmatitic gneisses as blocks and lenses of tens of meter size around the Haiyangsuo area, northeast part of Sulu UHP belt, eastern China. They comprise garnet-pyroxene granulites, eclogitized granulites and amphi-bolites. Their protolith compositions were mainly olivine tholeiite and quartz tholeiite, and show variation from Mg-rich to Fe-rich component as tholeiitic cumulates. Pearce's element ratio slopes suggested that protolith of these rocks were comagmatic, and generated from a primary magma by fractional crystallization of plagioclase, olivine and clinopyroxene. The crystallization differentiation has also been evidenced by trace elements, such as parallel REE patterns, Ni vs Ce variations, Sr increasing depletions, although the large ion lithophile elements (LILE) were modified to different extent during metamorphism. Trace element composition and Nd isotopes indicate a depleted mantle origin for these rocks. But they are not likely to be fragments of ophiolites or tholeiites connected with subduction, they formed probably at intra-continent environment. Sm-Nd whole rock isochron age of 2252±180Ma indicates approximately the formation age of igneous protolith of these rocks, almost 2000Ma earlier than the formation of the Dabie-Sulu UHP collision zone at about 240-220 Ma.
Protolith
Lile
Fractional crystallization (geology)
Pyroxene
Cite
Citations (0)
This thesis summarises the results of four original papers concerning U-Pb geochronology and geochemical evolution of Archaean rocks from the Kuhmo terrain and the Nurmes belt, eastern Finland. The study area belongs to a typical Archaean granite-greenstone terrain, composed of metavolcanic and metasedimentary rocks in generally NS trending greenstone belts as well as a granitoid-gneiss complex with intervening gneissic and migmatised supracrustal and plutonic rocks. U-Pb data on migmatite mesosomes indicate that the crust surrounding the Tipasjarvi-Kuhmo-Suomussalmi greenstone belt is of varying age. The oldest protolith detected for a migmatite mesosome from the granitoid-gneiss complex is 2.94 Ga, whereas the other dated migmatite protoliths have ages of 2.84–2.79 Ga. The latter protoliths are syngenetic with the majority of volcanic rocks in the adjacent Tipasjarvi-KuhmoSuomussalmi greenstone belt. This suggests that the genesis of some of the volcanic rocks within the greenstone belt and surrounding migmatite protoliths could be linked. Metamorphic zircon overgrowths with ages of 2.84–2.81 Ga were also obtained. The non-migmatised plutonic rocks in the Kuhmo terrain and in the Nurmes belt record secular geochemical evolution, typical of Archaean cratons. The studied tonalitic rocks have ages of 2.83–2.75 Ga and they have geochemical characteristics similar to low-Al and high-Al TTD (tonalite-trondhjemite-dacite). The granodiorites, diorites, and gabbros with high Mg/Fe and LILE-enriched characteristics were mostly emplaced between 2.74–2.70 Ga and they exhibit geochemical characteristics typical of Archaean sanukitoid suites. The latest identified plutonic episode took place at 2.70–2.68 Ga, when compositionally heterogeneous leucocratic granitoid rocks, with a variable crustal component, were emplaced. U-Pb data on migmatite leucosomes suggest that leucosome generation may have been coeval with this latest plutonic event. On the basis of available U-Pb and Sm-Nd isotopic data it appears that the plutonic rocks of the Kuhmo terrain and the Nurmes belt do not contain any significant input from Palaeoarchaean sources. A characteristic feature of the Nurmes belt is the presence of migmatised paragneisses, locally preserving primary sedimentary structures, with sporadic amphibolite intercalations. U-Pb studies on zircons indicate that the precursors of the Nurmes paragneisses were graywackes that were deposited between 2.71 Ga and 2.69 Ga and that they had a prominent 2.75–2.70 Ga source. Nd isotopic and whole-rock geochemical data for the intercalated amphibolites imply MORB sources. U-Pb data on zircons from the plutonic rocks and paragneisses reveal that metamorphic zircon growth took place at 2.72–2.63 Ga. This was the last tectonothermal event related to cratonisation of the Archaean crust of eastern Finland.
Protolith
Migmatite
Greenstone belt
Hadean
Cite
Citations (5)
Extremely 18O-depleted zircons from granitic gneisses, with δ 18O values as low as -7.8‰, were found in the Zaobuzhen area in the Weihai region, at the northeastern end of the Sulu orogen. SHRIMP zircon U-Pb ages and the oxygen isotope compositions were determined for the low δ 18O zircons. CL images reveal that the low δ 18O zircons are primarily of igneous origin, but some grains underwent metamorphic recrystallization. The igneous zircons from a granitic gneiss sample yield a concordant U-Pb age of (760±49)Ma and an upper intercepted age of (751±27)Ma, indicating a protolith of Middle Neoproterozoic age. Metamorphic zircons from the same sample yield a concordant U-Pb age of (232±4)Ma and a lower intercepted age of (241±33)Ma, pointing to Triassic UHP metamorphism. Most of the igneous zircons have unusually variable δ 18O values of -7.76‰ to 5.40‰, indicating that the gneiss protolith was intruded as low δ 18O magma that was generated by the partial melting of altered rocks, which suffered intensive water-rock interaction with a low δ 18O fluid at high temperatures during the Neoproterozoic. The preservation of extreme 18O-depletion in the zircons suggests that there is no remarkable oxygen isotope exchange between the metagranite and the mantle during the processes of Triassic subduction and exhumation. The protolith nature, metamorphic timing and oxygen isotope compositions of the granitic gneisses in the Weihai region are similar to those of granitic gneisses in the Qinglongshan area in the southwestern part of the Sulu orogenic belt, indicating that the gneisses along the Sulu orogenic belt share the same nature of protolith origin, water-rock interaction and UHP metamorphism. The present study provides tight constraints not only on the origin of extremely 18O-depleted zircons, but also on the protolith nature of granitic gneisses in the Sulu orogenic belt.
Protolith
Cite
Citations (0)
Protolith
Lile
Fractional crystallization (geology)
Pyroxene
Cite
Citations (12)
Protolith
Cite
Citations (38)
The Cenozoic basalts from Hannuoba contain abundant xenoliths derived from mantle and crust.As pyroxenite and granulite xenoliths appear very similar by naked eye sometimes,it is easy to confound the two kinds of rocks.Here we present our research on the genesis of pyroxenite and granulite xenoliths through petrographical observation,mineral chemistry and trace element and Sr-Nd isotopic compositions of clinopyroxene.The results indicate that the two kinds of xenoliths are remarkably different.Granulite xenoliths have layered cumulate structure and both kinds of pyroxenes(salite and bronzite) are relatively rich in FeO and poor in MgO.Clinopyroxene in granulite xenolith is relatively enriched in REE and characterized by high 87Sr/86Sr ratios and low 143Nd/144Nd ratios.In contrast,pyroxenite xenoliths generally show massive structure.Diopside and bronzite-enstatite of the pyroxenite xenoliths are relatively rich in MgO and poor in FeO.Clinopyroxene in pyroxenite xenolith shows very low REE content and a REE-depleted pattern.Isotopic compositions of pyroxenite xenoliths are between those of peridotite and granulite xenoliths.The above features as a whole suggest that granulite xenoliths result from magma underplating,which was subjected to lower crustal contamination.However,the pyroxenite xenoliths,distinguished from peridotite and granulite xenoliths,are derived from enriched mantle.
Xenolith
Peridotite
Underplating
Cite
Citations (3)