V43C-1547
Continental Crust Recycling: a Numerical Modeling Prespective
In the recent years, an increasing number of geochemical evidences of continental crust (cc) contributions in OIB lavas have been reported worldwide, suggesting that part of the lithospheric continents have sunk and were recycled in the deep mantle. So far, crustal recycling processes have been identified as eclogitization of the lower crust, foundering and subduction of (or at least part of ) continents. We perform 2D numerical models of subduction under a fix continent of a 1) retreating and 2) advancing oceanic slab attached to a continental passive margin to test this last process. 1)If the slab is retreating, when collision occurs and the continents are poorly coupled, asthenospheric mantle wedges between the continents triggering the retreating and delamination of the converging continental plate. A discrete volume of cc (mainly the lower crust) is then dragged into the deep mantle indicating that this is an efficient process for crustal recycling. 2)If the plates are coupled and convergence does not stop after collision, a large volume of lower cc is transported in the deep mantle. In both cases, the rheologically weak part of the crust (upper crust and sediments, Wet Quartzite flow law) is scraped off and accreted to the accretionary wedge. Results then show that cc recycling occurring after collision is a considerable process that has to be taken into account for both crustal mass balancing and crustal growth models.
V43C-1548
Recycled Lower Continental Crust: Evidence From Peridotite Xenoliths in Mesozoic High-Mg Diorites of the Eastern North China Craton
Recycling of continental lithosphere, together with recycling of oceanic crust, has been proposed to play a key role in plate tectonics, plume magmatism, crustal evolution and formation of chemical heterogeneities within the mantle. However, direct evidence for recycling of the deep continental lithosphere into the mantle is scarce (Gao et al., 2004). Petrological and geochemical data of the harzburgite-dunite-wehrlite-orthopyroxenite xenoliths hosted in Early Cretaceous high-Mg diorites of the North China Craton (NCC) provide direct evidence for recycling of the deep continental lithosphere into the mantle. Firstly, the veined and zoned orthopyroxenes (i.e., orthopyroxenite) surrounding chromite, together sometimes with phlogopite, can be found in spinel-harzburgite and chromite-bearing dunite, implying that an adakitic melt-peridotite interaction had taken place. Additionally, the residual olivine can be found in poikilitic clinopyroxene (1-2 mm) from wehrlite, also suggesting the introduction of a melt into the peridotite. Secondly, the olivine and orthopyroxene from the harzburgite and dunite xenoliths can be subdivided into two groups based on their Mg#'s (where Mg#\ = molar 100 ´ Mg/(Mg + Fetot)), i.e., high-Mg#\ group (89-95, averaging 91.4) and low-Mg#\ group (82-87, averaging 85). The former should be residues by large degrees of polybaric melting, the latter could be attributed to the reaction of an adakitic melt and peridotite . In addition, veined and zoned orthopyroxenes from dunites have much higher rare earth element (REE) abundances than primary orthopyroxenes from harzburgite, whereas poikilitic clinopyroxenes from wehrlite are characterized by strong enrichment in light rare earth elements (LREEs) and depletion in heavy rare earth elements (HREEs). These results indicate that the introduction of an adakitic melt had occurred in the previous depleted mantle peridotites. Thirdly, the dunite, harzbugite, and wehrlite xenoliths display "U" type of REE patterns on chondrite- normalized diagrams, implying that they could have been subjected to refertilization by late metasomatism. The initial 87Sr/86Sr ratios and ε Nd(t) values for the xenoliths range from 0.7058 to 0.7212 and +0.18 to - 10.65, respectively. Taken together, these results, combined with the strong depletion in HFSE and existence of the Archean inherited zircons in the host rocks, suggests that the previous depleted mantle peridotites had been subjected to modification of an adakitic melt, whereas the delaminated NCC lower continental crust is the most likely candidate for the source material of the adakitic melts that appear to be the primary metasomatic agent observed in these xenoliths (Xu et al., 2006). References Gao S. et al. (2004) Nature 432: 892-897. Xu W. L. (2006) Geology 34: 721-724.
V43C-1549
Geochemistry of the High-Mg Andesites at Zhangwu, Western Liaoning: Implication for Delamination of Newly Formed Lower Crust
Ten volcanic samples at Zhangwu, western Liaoning Province, North China were selected for a systematic geochemical, mineralogical and geochronological study, which provides an opportunity to explore the interaction between the continental crust and mantle beneath the north margin of the North China craton. Except one basalt sample (SiO2= 50.23%), the other nine samples are andesitic with SiO2 contents ranging from 53% to 59%. They have relatively high MgO (3.4%-6.1%, Mg# =50-64) and Ni and Cr contents (Ni 27ppm- 197ppm, Cr 51ppm-478ppm). Other geochemical characteristics of Zhangwu high-Mg andesites (HMAs) include strong fractionation of light rare earth elements (LREE) from heavy rare earth elements (HREE), and Sr from Y, with La/Yb greater than 15, and high Sr/Y (34-115). Zircons of andesite YX270 yield three age groups with no Precambrian age, which precludes origin of the Zhangwu HMAs from the partial melting of the Precambrian crust. The oldest age group peaking at 253 Ma is interpreted to represent the collision of the Siberia block and the North China block, resulting in formation of the Central Asian orogenic belt by closure of the Mongol-Okhotsk Ocean. The intermediate age group corresponds to the basalt underplating which caused the widespread coeval granitoids in the North China craton with a peak 206Pb/ 238U age of 172 Ma. The youngest age group gives a 206Pb/ 238U age of 126 Ma, which is interpreted as the eruption age of the Zhangwu HMAs. The high 87Sr/86Sri(126Ma)>0.706 and low ε Ndi(126Ma) = -6.36~-13.99 of the Zhangwu HMAs are distinct from slab melts. The common presence of reversely zoned clinopyroxene phenocrysts in the Zhangwu HMAs argues against the origin of the Zhangwu HMAs either from melting of the water saturated mantle or melting of the lower crust. In light of the evidence mentioned above, the envisaged scenario for the formation of the Zhangwu HMAs is related to the basaltic underplating at the base of the crust, which led to the thickening of the lower crust and formation of lower crustal eclogite, followed by foundering of the eclogitic lower crust into the asthenosphere. The foundered eclogite then melted and the resultant melts interacted with surrounding peridotite during their upward transport, which finally produced the high-Mg andesites. This well explains the high-Mg adakitic characters and absence of ancient inherited zircon in the Zhangwu lavas.
V43C-1550
Potassic Volcanic Rocks from the Shandong Province and its Implication to the Mesozoic Crustal Delamination in East China
Two successions of Mesozoic volcanic rocks of Qingshan Group were sampled from the eastern- and western Shandong Province, eastern North China craton. The Mengyin volcanic suite from the western Shandong were dated at 128 2 Ma by zircon U-Pb method using LA-ICPMS. The volcanic rocks are dominated by high-K to shoshonitic andesite, and are further divided into two subgroups by their Mg#. The high-Mg rocks are characterized by Mg#>60 (molar 100×Mg/(Mg+Fetotal)) and higher Cr and Ni along with evidently higher content in incompatible elements relative to the low-Mg rocks (Mg#<60). Coherent variations on Harker diagrams between the two subgroup samples are lacking. The Jiaonan volcanic suite from the eastern Shandong gives zircon U-Pb ages of 106 2 and 98 1 Ma for lower button and top of the succession, respectively, and consists of high-K alkali rhyolite and shoshonitic andesite, typical of bimodal igneous suite. In addition to characters of high content in MgO, Cr and Ni, the Jiaonan mafic rocks are also featured by distinctively higher content in incompatible elements compared with their intercalated felsic lavas. To observations of lacking elemental co-variation on the Harker diagrams and the remarkably higher incompatible elemental contents of relatively primitive rocks (Mg#>60) relative to those of Mg# < 60 of the Mengyin suite, and the andesitic relative to the felsic samples of the Jiaonan suite, it is difficult to interpret the petrogenesic correlations of the high-Mg and low-Mg rocks in Mengyin suite, and the andesitic- and rhyolitic lavas in Jiaonan succession by crystal fractionation of parental magmas. Inherited zircons ranging of 3242 to 1846 Ma are found in the Mengyin lavas. Along with Nd isotopic model ages (TDM) of 1.8 to 2.6 Ga for the Mengyin suite and 1.7 to 2.8 Ga for the Jiaonan suite, it suggests that the two volcanic successions were sourced by old lower crustal rocks. Trace element modeling using the batch melting model indicates that the average Mengyin low-Mg andesite can be explained as product of ~35% partial melting of the average Archean mafic lower crust rock of the North China craton (Gao et al., 1998). However, high content in MgO, Cr and Ni as well as incompatible elements for Mengyin high-Mg andesitic and the Jiaonan mafic lavas need additional process to account for. Such an elemental enrichment feature can be interpreted by an interaction between the magmas sourced from the Archean lower crust and mantle ultramafic rocks, resulting increasing MgO, Cr and Ni contents coupled with higher contents in incompatible elements for the remained magmas through consuming melts during their interaction with the mantle rocks (Rapp et al., 1999). Thus, it suggests that the Mesozoic mafic volcanic lavas sourced by the Archean lower crust experienced a magmatic evolution process characterized by ascending through and interaction with the lithospheric mantle, indicating a petrogenesis of lower crust delamination.
V43C-1551
Paired Magmatic Belts and Multiple Orogenesis: a Case Study of the Central China Orogen
Many orogens on Earth, which are developed in long-lived (>300 Ma) mobile belts, have undergone multiple orogenesis. Hyndman et al. (2005) proposed that these mobile belts had been evolved from backarcs, where extensional rifting had resulted in the thinning of lithosphere and thermal convection, which in turn led to the heating of backarcs. When continents collided, deformation took place exactly in the hot and softened backarc regions. Orogenic heat responsible for magmatic intrusion, high grade metamorphism and ductile deformation was largely derived from the early-formed hot backarc lithosphere rather than the deformation of the orogens themselves. The searching for the ¡°paired magmatic belts" is critical for the understanding of the mechanism and processes of multiple orogenesis. The so-called paired magmatic belts are termed as two parallelly distributed and similarly aged magmatic belts that are formed in association with island arc-continental arc and backarc extension. While there is common occurrence of ¡°paired magmatic belts" in orogens around the world, the Paleozoic ¡°paired magmatic belts" of the Central China Orogen including the Qinling-Dabie Orogen has provided us with a typical case. Within the northern portion of the Central China Orogen, i.e., the Beihuaiyang- North Qinling-Qilian ranges, Early Paleozoic intrusions are widely distributed. They are largely dated as 460- 430Ma. The rock types range from mafic to acidic intrusions, in which gabbros, (quartze) diorite and granite dominates. Petrographic and geochemical features suggest that the magmatic belt is related to subduction of the Paleo-Qinling Ocean Plate. In the southern margin of the Central China Orogen, Mid-Late Paleozoic mafic dyke swarms and bimodal alkaline dyke swarms are well developed. These rocks are mostly dated as 440-390 Ma. This magmatic belt stretches westward from Suizhou in the southern piedmont of Dabieshan to the southern margin of the Qaidam block and corresponds to the back-arc extension resulted from the subduction in the north. The bimodal alkaline rocks are enriched in large ion lithophile elements and high field strength elements, sharing features with rifting-related bimodal suit. The Suizhou alkaline granite in the southern piedmont of Dabieshan is a representative of these extension-related plutons. The two Paleozoic magmatic belts of the Central China Orogen are of distinct nature but similarly aged and parallelly distributed, suggesting that they were formed in the same dynamic system. Here we envisage a model to explain the occurrence of the Paleozoic paired magmatic belts. In the Ordovician-Silurian period, the subduciton of the Paleo-Qinling Oceanic plate underneath the Yangtze craton have resulted in roughly simultaneous back-arc extension in the southern margin of the Dabie-Qinling. These processes led to the formation of the paired magmatic belts in association with subduciton and backarc extension, respectively. As an Early Paleozoic backarc, the Mianlue-Suizhou belt in South Qinling-South Dabie continued extension in the Late Paleozoic, leading to the heating, thinning and softening of lithosphere and even the local occurrence of oceanic crust and subduction. These facilitate the collisional orogenesis, crustal deformaiton and deep subduction along the Mianlue-Suizhou belt in the Early Mesozoic, and are critical to the composite orogenesis of the Central China Orogen.
V43C-1552
Geochemical Consequences of Lithospheric Delamination in the Eastern Mediterranean: Evidence From Young Turkish Basalts
Magmatism associated with continental collision is increasingly attributed to major disturbance of or within the lithosphere. Geochemical and isotopic data on post-collisional primitive mafic lavas from across the Anatolian plate enable us to assess the effects of lithospheric delamination (slab rollback and breakoff) as indicated by geophysical studies. The Anatolian province displays geodynamically complex manifestations of the closure of neo-Tethys and the collision between Africa and Europe that commenced circa 30 Ma. The current south- southwestward motion of Anatolia, a.k.a. "Turkish escape", is accommodated by slab rollback along the Hellenic trench and orogenic collapse along both the eastern and western margins of the microplate. Volcanism occurs primarily along the fault zones that border and cross-cut Anatolia, and major element characteristics of the lavas vary with both space and time. In Western Anatolia, early Miocene collisional calc- alkaline magmatism was followed by Quaternary alkaline volcanism (Alici et al. 1998, 2002; Aldanmaz et al. 2000, 2006) related to orogenic collapse, presumably resulting from slab rollback. Orogenic collapse in Eastern Anatolia is facilitated by slab breakoff as determined by geophysical studies (Zor et al. 2003; Angus et al. 2006; Lei & Zhao 2007). This transition was accompanied a change from mid-Miocene calc-alkaline to Quaternary alkaline volcanism (e.g., Yilmaz 1990; Pearce et al. 1990). Central Anatolia displays calc-alkaline and tholeiitic volcanism, including alkali olivine basalts; plate tectonic reconstructions (Lyberis et al. 1992) indicate that the African slab did not reach Central Anatolia. Sr-Nd isotope values from each volcanic province define linear arrays that converge upon a common unradiogenic value typical of global depleted mantle. We suggest that mafic volcanism throughout Anatolia is supported by a common asthenospheric component, modified by identifiable, location-specific additions. In areas of lithospheric removal, young basalts have Sr-Nd values close to that of the inferred asthenospheric source, whereas older lavas display more enriched signatures. Limited Pb isotopic data fall within the range of Atlantic and Pacific Ocean sediments, suggesting the mantle signature is masked or strongly influenced by sediments. We focus on the Sivas volcanics, northernmost of the Central provinces, where the most highly magnesian lavas of Anatolia are found. Incompatible trace element considerations suggest that the Sivas suite provides key insights into the nature of the common source region. These lavas have MORB- and OIB-like values of most incompatible trace elements e.g., La/Nb, Ba/Nb, Ba/Rb, Rb/Sr and Th/La, and lack positive Pb anomalies characteristic of crustal interaction. Their geochemical and isotopic compositions - and comparisons between Sivas and areas of orogenic collapse to the east and west - provide new information on the interaction between lithospheric and asthenospheric materials across Turkey. References: Aldanmaz et al. 2000 JVGR 102, 67-95; Aldanmaz et al. 2006 Lithos 86, 50-76; Alici et al. 1998 JVGR 85, 423-446; Alici et al. 2002 JVGR 115, 487-510; Angus et al. 2006 GJI 166, 1335-1346; Lei & Zhao 2007 EPSL 257, 14-28; Lyberis et al. 1992 Tectonophysics 204, 1-15; Pearce et al. 1990 JVGR 44, 189-229; Yilmaz 1990 JVGR 44, 69-87; Zor et al. 2003 GRL 30, doi: 10.1029/2003GL018192
V43C-1553
Geochemistry of a new Enriched Mantle Type Locality in the Northern Hemisphere: Implications for the Origin of the EM-I Source
Late Cretaceous nephelinitic volcanic rocks from Godzilla seamount in the eastern North Atlantic (34°N latitude) have trace element and Sr- Nd- Pb- Hf isotope compositions similar to the Enriched Mantle I (EM-I) endmember, except for their low 207Pb/204Pb relative to 206Pb/204Pb ratios (206Pb/204Pbin=17.7, 207Pb/204Pbin=15.34) plotting below the Northern Hemisphere Reference Line on the uranogenic Pb isotope diagram. O isotope data on clinopyroxenes and amphiboles (δ18O amphibole= 5.6-5.8‰) are mantle-like. Age and location of the isolated Godzilla seamount, however, preclude it being derived from either the Madeira or Canary hotspots and therefore a derivation from a shallow (lithospheric/asthenospheric) melting anomaly is proposed. As observed in mid-ocean ridge and ocean island basalts, there is a systematic decrease of 207Pb/204Pb ratios (and Δ7/4) in the individual EM-I endmember type localities towards northern latitudes with Godzilla lying on the extension of this trend. This trend is mirrored in enriched subcontinental lithospheric mantle inferred from ultra- potassic volcanic rocks, such as lamproites and kimberlites and therefore a global pattern in 207Pb/204Pb ratios and Δ7/4 is suggested. The geochemical composition of Godzilla lavas as well as other EM-I endmember type localities and the enriched (DUPAL) anomaly in the southern hemisphere could reflect derivation from ancient, metasomatized subcontinental lithospheric mantle. We propose a two-stage model to explain the trace element and isotopic composition of the EM-I mantle endmember localities worldwide: 1) During the early history of the Earth, subcontinental lithosphere was metasomatized by melts from subducted slabs along convergent margins generating low μ (238U/204Pb) sources, and 2) as the Earth cooled, hydrous fluids replaced hydrous melts as the main slab component metasomatizing the subcontinental lithospheric mantle (generating EM-I sources with higher μ). In accordance with this model, the global variations in 207Pb/204Pb ratios and Δ7/4 could reflect geographic differences in μ and/or the age at which the transition from stage 1 to 2 took place in the Archaean lithosphere. This model would require re-definition of the EM-I endmember to low 206Pb/204Pb, high 208Pb/204Pb (positive Δ8/4) but variable 207Pb/204Pb (positive or negative Δ7/4).
V43C-1554
Caesium as a geochemical tool to define crustal-mantle interaction processes within North Atlantic LIP
In the ongoing 'Plume Debate' a major unsolved problem, is the contribution from the continental lithosphere to the production of melt and to the contamination of mantle melts during Large Igneous Province (LIP) magmatic episodes. Divergent opinions originate mainly from the difficulties in distinguishing trace element and isotopic characteristics of "fresh" continental lithospheric signatures from those acquired by involvement of recycled lithosphere within an ascending plume. This is an inherent limitation of our geochemical tools. As a result, a better characterization of LIP magmas and alternative petrogenetical approaches are needed to ultimately clarify which of the observed variations is due to a deep, respectively shallow mantle source. To this end, this study has used Caesium, along with traditional geochemical tools (trace elements and radiogenic isotopes) to investigate the potential influence of continental crustal contamination / assimilation processes on LIP magmas. Cs with its [Xe] 6s1 electron configuration is the most "lithophile" of all elements. This trace element shows the greatest contrast in abundance between mantle melts, lower crust and especially upper crust. In geodynamic earth models Cs is always strongly enriched in the continental crust and depleted in all possible geochemical reservoirs within the earth's interior. During subduction Cs should be liberated very early on from the subducting slab during dehydration processes. As a result, Cs concentrations in magmas are extremely sensitive to continental crust / mantle interactions. A systematic investigation of Cs from mid-Norwegian Volcanic Rifted Margin rocks (ODP Leg 104) defined a decreasing influence of continental crust during the excess magma formation. The Cs signature of the continental rift related ('Lower Series') rocks (6.13 to 0.12 ppm) confirms the crustal anatectic contribution of these rocks, and the data of the rift to drift transition ('Upper Series') tholeiitic MORBs (0.14 to 0.01 ppm) indicate that a minor amount of crustal material might have been involved during the SDRS formation. Due to positive correlations of Cs with both, Sr and Nd isotopic systems it is unlikely that the enhanced Cs content reflects redistribution of Cs, e.g. by secondary seawater alteration processes. The correlation of enriched classical isotopic signatures with the Cs data, points to a major continental source for this enrichment. Project supported by EUROMARGINS and RM founded by BFR 05/133
V43C-1556
Continental Arc subduction Zone Processes: Insights from Quartz-Bearing High Mg# Metasomatic Selvages in Mantle Xenoliths
Subduction zones are fundamental in generating continental crust. An important step in the formation of an arc is the passage of slab derived fluids or melts through the mantle wedge. However, access to the mantle wedge or sub-arc lithosphere is generally rare in such environments. Here, we examine xenoliths from the Pleistocene Big Pine volcanic field in Owens Valley, California in effort to describe the metasomatic influences of the slab-derived fluids on the mantle underlying the Sierra Nevada batholith, the remnants of a Mesozoic continental arc. These mantle xenoliths are plagioclase-quartz-feldspar-clinopyroxene-phlogopite selvages and veins, which contain abundant accessory zircon and sphene. The plagioclases are calcic and the clinopyroxenes have high Mg#s greater than 85, which suggest equilibrium with very primitive magmas or at least magmas with relatively high Mg#. Parodoxically, however, the presence of quartz requires a more felsic, and hence, evolved magma. We thus interpret these lithologies to represent cumulates from a high Mg# felsic magma. Such magmas could have originated as partial melts of mafic lithologies, such as subducting oceanic crust, delaminating lower crust, or garnet pyroxenite veins. As these felsic melts pass through the mantle wedge or lithospheric mantle, they may have reacted with peridotite, allowing them to inherit high Mg#s. The phlogopites require a hydrous origin for these lithologies, and such evidence for water as well as subduction-like trace-element systematics suggest a subducting slab as the likely candidate for the origin of these melts. Quartz and sphene thermometry reveal that the crystallization temperatures were between 650° C and 750° C while the presence of plagioclase constrains the maximum pressure to 1 GPa, indicating these cumulates formed near or just below the Moho. Preliminary U- Pb dating of sphenes and zircons suggest these cumulates formed in the early Tertiary. Further geochronology of the zircons will be useful in better understanding the link between these xenoliths and the subducting slab.