Andrew Kerr | Cardiff University (original) (raw)

Papers by Andrew Kerr

The Bohai Bay Basin basement is mainly composed of Archean granitoid gneisses with minor supracru... more The Bohai Bay Basin basement is mainly composed of Archean granitoid gneisses with minor supracrustal rocks and is the largest basin in the Eastern Block of the North China Craton. Due to a cover of Mesozoic and Cenozoic strata, little is known about the age and crustal evolution of this basement. In this study we report new zircon SHRIMP and LA-ICP-MS U-Pb and Lu-Hf isotope data for drill core samples, including TTGs (granodiorite, tonalite gneiss, trondhjemite gneiss), granites (monzogranite, syenogranite) and a leptite, with the aim of revealing the Archean crustal evolutionary history of the Bohai Bay Basin basement. The U-Pb age of magmatic zircons from these granitoids reveals that basement rocks were mainly generated by two-stage events at ~3.1 Ga and ~2.5 Ga. The ε Hf (t) values of ~3.1 Ga magmatic zircons vary from +0.56 to +8.27, and their corresponding single-stage model ages range from 3.3 Ga to 3.0 Ga. The ε Hf (t) values of ~2.5 Ga magmatic zircons range from − 12.87 to − 0.07, their corresponding two-stage model ages range from 3.8 Ga to 2.9 Ga with most ages from 3.4 Ga to 3.0 Ga. The Hf isotopic characteristics show that the crustal growth of basement beneath Bohai Bay Basin occurred mainly between 3.4 Ga and 3.0 Ga, different from crustal accretion ages of 2.9-2.7 Ga on the periphery of the Bohai Bay Basin. However, both areas were reworked by the ~2.5 Ga tectono-thermal event. Integration of this new data from the basin basement with previous data, indicates that the Eastern Block of the North China Craton may be controlled by a mantle plume during the ~2.5 Ga period. The results from this study are significant in assessing the tectonic environment of the eastern basement in the North China Craton.

The Zhob Ophiolite, Balochistan, Pakistan, comprises imbricated thrust sheets of igneous, sedimen... more The Zhob Ophiolite, Balochistan, Pakistan, comprises imbricated thrust sheets of igneous, sedimentary, and metamorphic rocks. The sedimentary units are more widely developed and found around the ophiolitic blocks while ultramafic, mafic, and metamorphic units occur in three imbricated thrust blocks, and are thrust over the sedimentary rock units. The gabbro bodies intrude all three blocks. Petrographically, the gabbro can be divided into hornblende gabbro, pyroxene gabbro, and olivine gabbro. The gabbros are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE) relative to N-MORB and have a negative Nb anomaly. Their overall trace element signatures suggest an island arc tectonic setting. Low Mg # (42-56), and low Ni, Co, and Cr contents indicate that the parental magma of the gabbro fractionated in an upper-level magma chamber en route to intrusion. Incompatible trace element ratios show that the parental magma of these rocks was produced by 5-30% partial melting of a slightly depleted-enriched spinel lherzolite mantle source. It is suggested that Zhob Ophiolite was formed in a Late Jurassic-Cretaceous Ceno-Tethyan convergence zone along with several other arcs and back-arc basins including Muslim Bagh, Waziristan, Semail, Zagros, Chagai-Raskoh, and Kohistan-Ladakh.

We present new whole-rock geochemical, Sr-Nd-Pb isotopic, and zircon U-Pb and Hf isotopic data fo... more We present new whole-rock geochemical, Sr-Nd-Pb isotopic, and zircon U-Pb and Hf isotopic data for Early Cretaceous magmatic rocks (trachy-andesite, pyroxene diorite, diorite porphyrite, and dolerite) in the Wulian area of Shandong, eastern North China. These data are used to constrain the mechanism of crust-mantle interaction at the edge of the Sulu orogenic belt. The belt formed by the collision of the Yangtze Craton (YC) and the North China Craton (NCC) during the Mesozoic. New zircon U-Pb dating indicates that the magmatic rocks were formed during the Early Cretaceous (123-124 Ma). These rocks are characterized by moderate contents of SiO 2 (50.0-60.0 wt.%), MgO (3.3-5.6 wt.%), Cr and Ni with low Nb/U ratios (0.8-11.8), and high La/Nb (5.2-24.5) and Ba/La ratios (12.3-38.1). They are enriched in large ion lithophile elements, depleted in high field strength elements, and are characterized by high initial 87 Sr/ 86 Sr values (0.7079-0.7088) and low ε Nd ðtÞ (-20.6 to-14.6). The samples have relatively low initial 206 Pb/ 204 Pb (16.38-17.18), 207 Pb/ 204 Pb (15.38-15.48), and 208 Pb/ 204 Pb (37.24-37.83) values. The Sr-Nd-Pb-Hf isotopic characteristics of the samples are similar to those of mafic rocks in the Sulu orogenic belt, suggesting that they might have similar sources. It is clear that the magma source of the samples involves both crustal and mantle materials and so we propose a model for crust-mantle interaction at the edge of Sulu orogenic belt. In this model, the Yangtze plate subducted deep below the northwestern NCC during the Triassic and was trapped in the lithospheric mantle. In the early Cretaceous, lithospheric extension in combination with asthenospheric upwelling resulted in partial melting of the overlying lithospheric mantle. The magmatic rocks in the Wulian area were generated by mixing between melts of both the lithospheric mantle of the NCC and the residual lower crust of the YC. This study therefore provides significant information on crust-mantle interaction at a continental subduction zone.

Jamaica has a complex geological history with rocks belonging to the Cretaceous Caribbean Large I... more Jamaica has a complex geological history with rocks belonging to the Cretaceous Caribbean Large Igneous Province (CLIP) in the east and Cretaceous oceanic island arc rocks in the centre and west. We present a new geochemical dataset for the CLIP and correlate this dataset and previous datasets using radiolarians and planktic foraminifers to the geological timescale. The palaeontological dating indicates that two phases of plateau activity-'main' phase in the late Turonian-mid Coniacian (c. 92-87 Ma) and an 'extended' phase in the Coniacian to mid Campanian (c. 88-75 Ma). These phases are also seen in the Beata Ridge and on the Lower Nicaragua Rise. The geochemistry indicates that both phases are typical large igneous province plateau basalts. The 'main' phase has slightly more depleted light rare earth elements than the extended phase, indicating mantle source heterogeneity, and a (Sm/Yb) n >1 indicates a deeper average depth of melting for the 'main' phase. The association of the basalts with sediments containing specific microfossil assemblages clearly demonstrates the existence of these two magmatic phases in Jamaica.

Although it has been argued that sediment recycling plays an important role in the differentiatio... more Although it has been argued that sediment recycling plays an important role in the differentiation of the continental crust, boron (B) isotopic data does not support a direct input of subducted sediments into arc magmas. This raises questions about the viability of sediment recycling as a process in the differentiation of continental crust. Here, we report B isotopic data from Miocene-Quaternary lavas derived from different sources in the northern margin of Tibet. These lavas have high B contents and negative δ 11 B values close to those of continental sediments. Strongly peraluminous rhyolites have the highest B (93 to 1559 ppm) contents with negative δ 11 B (− 9.7 to − 17.9) values. Adakitic dacites and trachyandesites exhibit the lowest B (18 to 29 ppm) contents with markedly negative δ 11 B (− 12.0 to − 35.7) values whereas olivine leucitites have B (37.2 to 59.3 ppm) contents with negative δ 11 B (− 8.3 to − 15.6) values. These lavas also have enriched Hf-Pb-Nd isotopic compositions similar to those of sediments. This data, combined with numerical modelling and geophysical and tectonic data for Cenozoic continental subduction in the northern margin of Tibet, indicates that: (1) the strongly peraluminous rhyolites were generated by partial melting of mica-bearing continental sedimentary rocks subducted to the depth of mid-to-lower crust; (2) adakitic lavas were derived by partial melting of sediment-bearing thickened lower crust underwent dehydration and eclogites-facies metamorphism; and (3) olivine leucitites were generated by partial melting of enriched mantle metasomatized by sediment-bearing eclogite-facies crustderived melts. Thus, at continental convergent margins, continental subduction is an important mechanism for sediment recycling and the evolution of continental crust.

Geophysically identified low-velocity zones (LVZs) are widespread within the Tibetan lower-middle... more Geophysically identified low-velocity zones (LVZs) are widespread within the Tibetan lower-middle crust, but their nature and role in models of the development of the Tibetan Plateau remain controversial. The debate stems mainly from whether the Tibetan crust was hydrous and has therefore undergone substantial melting. Here, we identify hydrous crustal xenoliths incorporated into 28 Ma syenite porphyries from central-northern Tibet. These xenoliths indicate the former existence of a cold (T = 680-790 • C) and water-rich (1.10-1.50 wt%) crustal endmember at a depth of 14-40 km in the Tibetan lower-middle crust. Our new petrological evidence indicates the LVZs are partially molten layers that have existed within the Tibetan crust since at least 28 Ma. High-to ultrahigh-temperature metamorphism since the Miocene is inferred to have triggered widespread melting of the former hydrous crust, which would have promoted the development of LVZs and resulted in a flat Tibet.

In recent studies of crustal growth using global zircon Hf-O isotopic datasets, high-δ 18 O zirco... more In recent studies of crustal growth using global zircon Hf-O isotopic datasets, high-δ 18 O zircons are typically attributed to intra-crustal reworking during which very little juvenile mantle-derived magmas were added to the crust. Although arc magmas may originate from a high-δ 18 O mantle wedge, it has been difficult to decipher the contribution of high-δ 18 O mantle to zircon-saturated felsic magma due to superimposed intra-crustal processes. We address this issue by combining the data from high-δ 18 O zircon-bearing ultramafic cumulates and coeval lavas from a Cretaceous magmatic arc in southern Tibet. The cumulates mainly consist of different proportions of cumulus olivine and intercumulus amphibole. Amphibole analyses show a transition from increasing to decreasing Zr with increasing SiO 2 (50-74 wt.%) contents in the intercumulus melts, indicating zircon saturation in late-stage interstitial melts. The εNd(t) values (2.4 ± 1.4) of the apatite grains crystallized before and after zircon remain almost constant. Interstitial zircons have δ 18 O (6.1-7.2) values similar to the earliest crystallized olivine (δ 18 O = 6.3-7.1) in the cumulates. The coeval lavas may represent the intercumulus melts extracted from amphibole-rich cumulates at different depths. Both the lavas and cumulates were ultimately derived from high-δ 18 O arc mantle modified by small amounts (<12%) of subducted sediments, and crystallized zircon during intra-crustal magma evolution without involving crustal contamination or melting. These high-δ 18 O zircons therefore are not products of crustal reworking, but record crustal growth during their crystallization (110 ± 2 Ma). Our study shows that the combination of zircon and olivine oxygen isotopes for ultramafic to felsic rocks is more effective than zircon data alone in evaluating the role of crustal growth vs. reworking in an arc system. The implication is that global zircon-based crustal evolution models that attribute all high-δ 18 O zircons to crustal reworking may conceal recent crustal growth.

Eocene mafic magmatism in the Himalaya provides a crucial window for probing the evolution of cru... more Eocene mafic magmatism in the Himalaya provides a crucial window for probing the evolution of crustal anatexis processes within the lower plate in a collisional orogen. We report geochemical data from the earliest postcollision ocean-island basalt-like mafic dikes intruding the Tethyan Himalaya near the northern edge of the colliding Indian plate. These dikes occurred coeval, and spatially overlap, with Eocene granitoids in the cores of gneiss domes and were likely derived from interaction between melts from the lithosphere-asthenosphere boundary and the Indian continental lithosphere. We propose that these mafic magmas were emplaced along lithospheric fractures in response to lithospheric flexure during initial subduction of the Indian continent and that the underplating of such mafic magmas resulted in orogen-parallel crustal anatexis within the Indian continent. This mechanism can explain the formation of coeval magmatism and the geologic evolution of a collisional orogen on both sides of the suture zone.

The question of whether subducted continental crust can be recycled into post-collisional magmati... more The question of whether subducted continental crust can be recycled into post-collisional magmatism in continental collisional zones remains controversial. Post-collisional mantle-derived ultrapotassic rocks are widespread in western part of central Lhasa block (WCL) of the Tibet-Himalaya orogen and show arc-type trace-element signatures and very enriched Sr-Nd isotopes, which have been explained by the recycling of subducted Indian continental crust. However, due to the lack of late Cretaceous mafic magmatism in the WCL, the nature of the ancient subcontinental lithospheric mantle (SCLM) beneath the WCL has not been well constrained. It is therefore still unclear whether the enriched components of these ultrapotassic rocks were derived from subducted Indian continental crust or inherited from the ancient SCLM. Here we report the mafic dykes from the TangraYumco area in the WCL, which formed at ~90 Ma (whole-rock 40 Ar-39 Ar, and zircon and titanite U-Pb ages). The diabase-porphyrites and diorite-porphyrites are similar to intraplate Nb-enriched basalts and magnesium adakitic rocks, respectively. This rock association reveals the lithospheric foundering process. Furthermore, the diabase-porphyrites generated by the interaction between the ancient SCLM and asthenosphere provide important constraints on the nature of the ancient SCLM, which shows distinct Nd isotopes to the postcollisional ultrapotassic rocks. In addition, the NW Lhasa block was in a rear-arc environment since the late Cretaceous and thus this SCLM was not significantly influenced by the ongoing northward subduction of the Indus-YarlungZangbu Neo-Tethys ocean. Finally, we propose that the post-collisional ultrapotassic rocks cannot be sourced from the ancient SCLM, but instead were derived from a relatively depleted mantle (e.g., juvenile lithospheric mantle) metasomatized by subducted Indian continental crust. Thus, this study confirms the recycling of subducted continental materials in continental collisional orogens.

Numerous Late Carboniferous-Early Permian dykes are found in West Junggar and represent an import... more Numerous Late Carboniferous-Early Permian dykes are found in West Junggar and represent an important part of the Central Asian Orogenic Belt. In this contribution, we use these dykes to assess the tectonic regime and stress state in the Late Carboniferous-Early Permian. The West Junggar dykes are mainly diorite/dioritic porphyrite with minor diabase and were formed in 324-310 Ma. They have been divided into two groups based on their orientation, petrology and geochronology. Group 1 dykes mostly comprise WNW-striking dioritic porphyrite and NE-striking diorite with minor diabase and resemble the Karamay-Baogutu sanukitoid. They were probably formed from depleted mantle at a relatively high temperature and pressure with the addition of 1-2% sediment/sedimental partial melt and 0-5% trapped oceanic crust-derived melts. Group 2 dykes are ENE-striking and are similar to sanukite in the Setouchi Volcanic Belt. These dykes were also derived from depleted mantle at a shallow depth but high temperature with the addition of 2-3.5% sediment/sedimental partial melt. Magma banding and injection folds in dykes and host granitoids indicate magma flow. Paleostress analysis reveals that both groups of dykes were formed in a tensile stress field. Their emplacement is favoured by presence of pre-existing joints or fractures in the host granitoids and strata. We conclude that large-scale asthenosphere mantle upwelling induced by trapped oceanic slab-off can explain the magmatism and significant continental crustal growth of West Junggar during Late Carboniferous to Early Permian.

The uplift of the Tibetan Plateau during the Miocene is crucial to understanding continental defo... more The uplift of the Tibetan Plateau during the Miocene is crucial to understanding continental deformation processes and global climatic events. However, the eastern Tibetan Plateau remains poorly investigated. Mantlederived magmatism provides crucial insights into the deep dynamic processes and surface uplift of the plateau. In this paper we report the first discovery of north-south trending lamprophyre dikes from the Aduo Basin in the eastern Qiangtang terrane, eastern Tibet. Our new age data show that these lamprophyre dikes were generated in Mid-Miocene (15-13 Ma). This new discovery has bridged the Mid-Miocene mantle-derived magmatic gap in the Qiangtang terrane. Trace element and Nd-Sr isotopic data indicate that they were derived by partial melting of enriched subcontinental lithospheric mantle. Our study implies that the eastern Tibetan Plateau had its attained near-maximum elevation by the Mid-Miocene. Combined with previous research results, we propose that the entire plateau almost simultaneously reached its near-maximum elevation by the Mid-Miocene.

The petrogenesis of continental arc magmas provides critical insights into thermal evolution and ... more The petrogenesis of continental arc magmas provides critical insights into thermal evolution and geodynamics of the continental lithosphere, crust-mantle interaction, and deep dynamic processes. In this study, we report new zircon U-Pb ages along with isotopic and elemental whole-rock geochemistry, mineral chemistry, and Hf-O isotope data for the Kalaqigu diorites and monzogranites of the Chinese Wakhan Corridor, South Pamir. Zircon U-Pb dating indicates that the Kalaqigu pluton was emplaced in the Early Cretaceous (ca. 108-106 Ma). The diorites are geochemically characterized by low SiO 2 (51.9-54.5 wt%) and CaO (7.7-9.4 wt%) contents, but high MgO (5.3-8.3 wt%), Al 2 O 3 (12.8-16.8 wt%), and TiO 2 (0.6-1.1 wt%) contents as well as high Mg # (56-65) values. Thus, they are similar to high-Mg diorites: enriched in large ion lithophile elements (e.g., K, Sr, and Ba) and light rare earth elements, while depleted in high field strength elements (i.e., Nb, Ta, Zr, and Hf). Combined with negative ε Nd (t) (−6.9 to −14.0) and ε Hf (t) (−9.9 to −12.2), and high (87 Sr/ 86 Sr) i (0.7075-0.7086) ratios, these observations indicate that they originated from an enriched lithospheric mantle source. High δ 18 O zrn (7.49‰-9.01‰) values, in conjunction with relatively high 207 Pb/ 206 Pb and 208 Pb/ 206 Pb ratios, suggest that the source was modified by subducted

Subduction polarity reversal usually involves the break off or tearing of the downgoing plate (DP... more Subduction polarity reversal usually involves the break off or tearing of the downgoing plate (DP) along the continent-ocean transition zone, in order to initiate subduction of the overriding plate (OP) with opposite polarity. We propose that subduction polarity reversal can also be caused by DP-OP coupling and can account for the early Paleozoic geological relationships in the Western Kunlun orogenic belt in the northwestern Tibetan Plateau. Our synthesis of elemental and isotopic data reveals transient (∼2 m.y.) changes in the sources of early Paleozoic arc magmatism in the southern Kunlun terrane. The early-stage (ca. 530-487 Ma) magmatic rocks display relatively high ε Nd (t) (+0.3 to +8.7), ε Hf (t) (∼3.6 to +16.0), and intra-oceanic arc-like features. In contrast, the late-stage (485-430 Ma) magmatic rocks have predominantly negative ε Nd (t) (∼4.5 to +0.3), ε Hf (t) (∼8.8 to +0.9), and higher incompatible trace elements (e.g., Th), similar to the sub-continental lithospheric mantle beneath the Tarim craton. This abrupt temporal-spatial variation of arc magmatism, together with the detrital zircon evidence, indicate that subduction polarity reversal of the Proto-Tethys Ocean occurred in a period of ∼10 m.y., consistent with the time interval reflected by ophiolite age. This rapid polarity reversal corresponds with the absence of ultrahigh-pressure (UHP) metamorphic and post-collisional magmatic rocks, features normally characteristic of slab break-off or tearing. Numerical modeling shows that this polarity reversal was caused by plate coupling during arc-continent collision. This coupling modified the normal succession of arc-continent collision events, preventing slab break-off or tearing-induced buoyant rock rebound and asthenosphere upwelling. Our model successfully explains early Paleozoic orogenesis in the Western Kunlun orogenic belt and may be applied elsewhere where post-collisional magmatic and UHP rocks are absent.

In-situ U-Pb ages and Hf-O isotopes of zircon are frequently used to study the petrogenesis of ma... more In-situ U-Pb ages and Hf-O isotopes of zircon are frequently used to study the petrogenesis of mafic microgranular enclaves (MMEs) in granitoids. However, whether zircons in MMEs have recorded the primary isotopes of their host MMEs is still questionable. In this study, MMEs and host granites from the Cuojiaoma granitic pluton in the Triassic Yidun arc belt, eastern Tibetan Plateau, are studied. LA-ICP-MS zircon U-Pb dating of studied MMEs yields an age of 213 ± 2 Ma. Model calculations indicate that the whole-rock composition of the MMEs was different from the melt composition from which the zircon grains grew. The MMEs crystallized during magma mixing based on analyses of zircon trace element and Zr-Hf isotopes. None of zircons in this study have recorded the primary isotope signature of the mafic endmember of MMEs because they are different from the host granite in terms of zircon morphology, εHf values and ΔQFM. The Zr isotopic data demonstrate open-system magmatic crystallization for the host granite and that magma mixing for MMEs play key role in controlling Zr isotopic variations. Initial 87 Sr/ 86 Sr ratios of An-rich plagioclase crystals in groundmass from MMEs range from 0.7027 to 0.7058, which are significantly lower than host granites (0.7080-0.7089). The lowest initial 87 Sr/ 86 Sr ratios (0.7027-0.7028) are also lower than arc volcanic rocks and similar to a depleted mantle. Combined with previous studies, we propose that MMEs were generated during post-collision extension by mixing of mafic magma of Xiaxiaoliu basalt and felsic magma from host granite.