Shallow Emplacement Depth Research Articles

Secular change in the nature of mantle and tectonic evolution of northwestern margin of the Yangtze Block during Neoproterozoic: Constraints from the mafic intrusions and associated granitoids of the Hannan and Xiaomoling complexes

To reveal the Neoproterozoic tectonic evolution of the northwestern margin of the Yangtze Block, a petrological and geochemical study was carried out for the mafic intrusions and associated granitoids from the Hannan and Xiaomoling Complexes. The ca. 900–880 Ma Xiaomoling diorite and granodiorite have high K2O contents (2.29–3.57 wt%) and belong to the high-K calc-alkaline series. They display depleted whole-rock Sr–Nd–Hf isotope compositions (0.70151–0.70398, +0.6 to +2.5 and +4.4 to +11.2, respectively) and positive zircon εHf(t) (+3.6 to +9.9) with slightly elevated bulk zircon δ18O values (6.4–6.8 ‰) and minor Paleoproterozoic (∼2.2 Ga) inherited zircons, indicative of derivation from remelting of juvenile mafic crust with crustal contamination in a continental arc setting. The ca. 866 Ma Hannan gabbro-diorites show enrichments in LILEs and LREEs and depletions in HFSEs and display depleted Sr–Nd–Hf isotope compositions (0.70267, +5.4 and +17.8, respectively), positive zircon εHf(t) values (+8.6 to +12.0) and mantle-like zircon δ18O values (5.0–6.2 ‰), suggesting that they were generated through flux melting of a depleted mantle wedge. The ca. 850–840 Ma Xiaomoling hornblende gabbro and diabase display depleted Sr–Nd–Hf isotope compositions (0.70279–0.70288, +2.7 to +3.2 and +9.8 to +10.5, respectively) and are characterized by high Nb/U ratios and less enrichment in LREEs and LILEs, indicating that they were formed by decompression melting of a depleted mantle wedge in a back-arc setting. However, the coeval high-Mg diorite displays enriched whole-rock Sr–Nd–Hf isotopes (0.70477–0.70701, −4.0 to −6.0 and −7.2 to −7.4, respectively), negative zircon εHf(t) values (−1.6 to −9.2) and elevated zircon δ18O values (5.3–7.5 ‰), reflecting that the mantle-derived melts have underwent various degrees of crustal contamination. The ca. 780 Ma Hannan gabbro is characterized by earlier crystallization of plagioclase than clinopyroxene, implying that their parental magma is anhydrous. Additionally, the relatively shallow emplacement depth and tholeiitic affinities imply that they were generated by mantle decompression melting probably associated with a continental rift environment in response to Rodinia breakup. Taken together, the secular changes in the geochemical and isotopic compositions of these mafic rocks in the northwestern margin of the Yangtze Block record the evolution from a continental arc at ca. 900–880 Ma to continental rifting at ca. 780 Ma.

Burial and exhumation of late paleozoic arc-related rocks in the Tulasu basin, Western Chinese Tianshan: Implication for the preservation of epithermal deposits in old orogenic belts

Epithermal deposits are susceptible to erosion owing to their shallow emplacement depth of 1 to 2 km, resulting in the observation that the majority of epithermal deposits worldwide are generally Cenozoic in age. Several large and super-large epithermal deposits likely associated with Late-Devonian to Carboniferous arc magmatism were recognized in the Tulasu volcanic-clastic sedimentary basin, western Chinese Tianshan. In order to further clarify the preservation processes of these old epithermal orebodies, apatite fission track (AFT) analysis has been undertaken in this study. Our AFT results gave a range of ages from 54.7 ± 6.3 Ma to 272.3 ± 8.7 Ma, and the mean confined fission track lengths are between 12.1 ± 0.2 μm and 13.5 ± 0.1 μm. Our new data, in combination with the 1:50,000 geological mapping, suggest that the ore-hosting Tulasu basin may have undergone multiple burial and exhumation episodes postdating the mineralization. The main burial scenario, probably occurring in the late Early Carboniferous (350–330 Ma) and Permian (300–250 Ma), may be related to subsidence on syn-volcanic faults concurrent with retreat of the Junggar Ocean and strong uplift of the neighboring mountain range induced by final closure of the Paleo-Asian Ocean, respectively. The calculated thickness of ca. 4.83 km of sedimentary rocks deposited during the burial events form a critical shield for the epithermal ore bodies that have been protected from substantial erosion by subsequent geological processes. Exhumation occurred in two episodes: Late Triassic-Cretaceous (ca. 240–100 Ma) and Oligocene (ca. 35 Ma). The Late Triassic-Cretaceous erosion is characterized by a slow exhumation rate (ca. 0.012 mm/y), corresponding to a total unroofing thickness of ca. 1.67 km. In contrast, the Oligocene event exhibits as a higher exhumation rate (up to ca. 0.057 mm/y), with removal of ca. 3.83 km of sedimentary cover. Therefore, the epithermal deposits have undergone a complex geological path from formation to final exposure. Initial burial and the following sluggish exhumation are two fundamental factors for the preservation of epithermal deposits in the old orogenic belts.

A ~1.4 Ga alkaline mafic sill from the Carletonville area: connection to the Pilanesberg Alkaline Province?

Abstract Numerous Mesoproterozoic alkaline intrusions belonging to the Pilanesberg Alkaline Province are present within the Transvaal sub-basin of the Kaapvaal Craton. The Pilanesberg Complex is the best-known example; it represents one of the world’s largest alkaline complexes, and is associated with a northwest-southeast trending dyke swarm that extends from Botswana to the southwest of Johannesburg. This paper documents the results of a petrological and geochemical study of a thin mafic sill (here referred to as an alkaline igneous body, AIB), which intrudes the ca. 2 200 Ma Silverton Formation close to the southernmost part of the Pilanesberg dyke swarm. The AIB has only been observed in cores from a borehole drilled close to Carletonville. It is hypocrystalline, containing randomly oriented elongated skeletal kaersutite crystals and 6 to 8 mm varioles mainly composed of radially oriented acicular plagioclase. These two textures are related to undercooling, probably linked to the limited thickness (70 cm) of the AIB coupled with a probable shallow emplacement depth. Ar-Ar dating of the kaersutite gives an age of ca. 1 400 Ma, similar to the age of Pilanesberg Complex. However, the AIB is an alkaline basaltic andesite and is thus notably less differentiated than the Pilanesberg Complex and some of its associated dykes, such as the Maanhaarrand dyke, for which we provide whole-rock geochemical data. Literature data indicate that the Pilanesberg dyke swarm also contains mafic hypabyssal rocks suggesting a link between the dyke swarm and the AIB. The AIB is characterized by strongly negative εNd and εHf, that cannot be related to crustal contamination, as shown by positive Ti and P anomalies, and the absence of negative Nb-Ta anomalies in mantle-normalised trace element diagrams. The AIB magma is interpreted to have been derived from a long-lived enriched, probably lithospheric mantle reservoir. The AIB thus provides important information on the magma source of the Pilanesberg Alkaline Province.

Contrasting whole-rock and mineral compositions of ore-bearing (Tongchang) and ore-barren (Shilicun) granitic plutons in SW China: Implications for petrogenesis and ore genesis

A common perception is that oxidized magma is critical for the formation of a porphyry Cu ± Mo ± Au ore deposit. We have used an ore-bearing pluton (Tongchang) and an ore-barren pluton (Shilicun) in the western rim of the South China block to test this idea and to determine other important controls on ore genesis. Zircon UPb ages indicate that the Tongchang and Shilicun plutons were emplaced at 36.3 ± 0.2 Ma and 35.2 ± 0.4 Ma, respectively, broadly coinciding with strike-slip faulting in the region and continental collision that occurred ~300 km to the west. These two plutons are all characterized by significant light REE enrichments and pronounced negative NbTa anomalies, similar to the bulk crust. Apatite separates from these two plutons all have elevated initial 87Sr/86Sr (Tongchang, 0.70690 to 0.70796; Shilicun, 0.70703 to 0.70726) and negative εNd(t) (Tongchang, −6.2 to −7.3; Shilicun, −4.5 to −5.8). The mean εHf(t) and δ18O of zircon from these plutons are −1.4 and 6.8‰ for Tongchang, and −0.3 and 6.5‰ for Shilicun. The Sr-Nd-Hf isotope compositions are all within the ranges of the Neoproterozoic mafic arc lower crust in the region. Whole-rock U-Nb-Ta systematics indicate that the Tongchang and Shilicun plutons originated from an amphibole-bearing source and a garnet-bearing source, respectively, implying a shallower and more H2O-rich mafic source for the former than the latter. Quartz trace element data indicate that the Tongchang pluton was emplaced at a shallower depth than the Shilicun pluton. Zircon Ce4+/Ce3+ and whole-rock V/Sc ratios, and apatite SO3 and MnO contents indicate that the parental magmas of both plutons are as oxidized as those of some porphyry Cu deposits in northern Chile, but the parental magma of the Tongchang pluton is less oxidized than that of the Shilicun pluton. Apatite Cl-F-OH systematics and higher abundant hydrous silicate minerals such as amphibole and biotite in the Tongchang pluton than the Shilicun pluton indicate that the parental magma of the Tongchang pluton has higher Cl and H2O content than that of the Shilicun pluton. Our results show that oxidized magma is important but this alone cannot make a porphyry Cu ± Mo ± Au ore deposit. High contents of H2O-Cl in magma and shallow depth of emplacement are also important.

Potassic alkaline granitoid magmatism in the northern margin of the Tarim Craton: First evidence of a back‐arc extensional environment

The tectonic setting of the northern Tarim Craton during the Palaeozoic is vital to our understanding of the subduction polarity of the paleo‐oceanic plates in the Tianshan Orogen and the accretion history of the south‐western Central Asian Orogenic Belt. We first identified granitoids intruding the Palaeoproterozoic metamorphic rocks in the Kurchu area in the northern Tarim Craton. The zircon LA‐ICP‐MS U–Pb dating of this rock indicates that its crystallization age is 418 Ma, highlighting a late Early Palaeozoic magmatic event. Compared with the contemporaneous granitoids in the southern margin of the Tarim Craton, the Kurchu granitoid has high K2O (6.17–7.25 wt.%) and high alkaline (Na2O + K2O = 9.48–10.56 wt.%) contents. The Rittmann index (σ) ranges from 3.53 to 5.68, and samples plot in the shoshonite series in the K2O–SiO2 diagram, indicating that the rock is a potassic‐alkaline granite (PAG). In addition, this granitoid shows high REE concentrations (264–817 ppm) with significant Eu anomalies and is depleted in Ba, Sr, Nb, Ta, Ti, and P but enriched in Zr and Hf. These geochemical characteristics and high (87Sr/86Sr)i (0.70857–0.70995) and low εNd(T) (−10.67 to −10.24) values of the Kurchu PAG indicate that this rock was derived from the partial melting of the crust. The diagenetic conditions of the Kurchu PAG are high temperatures, as recorded by higher zircon saturation temperatures (855–919°C), low diagenetic pressure, a less shallow emplacement depth recorded by lower Sr (<200 ppm) concentrations, and a low oxygen fugacity recorded by lower Eu* (0.17–0.89), higher Zr (>355 ppm) and Hf (>9.73 ppm) concentrations, and trace element contents of zircons. In various discrimination diagrams, all samples consistently plot in rift‐related areas. These geochemical and diagenetic features suggest that this magmatic event probably occurred in a back‐arc environment. Therefore, there is back‐arc basin occurring in the northern Tarim Craton in the Early Palaeozoic, which provides strong evidence that this region was an active margin probably much earlier than Silurian, necessitating the southward subduction of the South Tianshan Ocean.

The Late Permian highly fractionated I-type granites from Sishijia pluton in southestern Inner Mongolia, North China: A post-collisional magmatism record and its implication for the closure of Paleo-Asian Ocean

Whole-rock major and trace elements, zircon UPb geochronology and Hf isotopic data have been implemented for the felsic rocks from the Sishijia pluton in southeastern Inner Mongolia, North China, in order to determine their petrogenesis and futher constrain the termination of Paleoasian Oceanic subduction in southeastern Central Asian Orogenic Belt. Petrographic observation indicates that Sishijia granitoids mainly consist of syenogranites and porphyritic granites, with the major mineral compositions of alkaline feldspar, quartz, plagioclase and biotite, without any alkaline melanocratic or peraluminous minerals. Miarolitic structures are widely developed in these granitoids, indicating for the characteristics of shallow emplacement depth and rapid condensation. Zircon LA-ICP-MS UPb dating yields weighted mean ages of 256.2 ± 1.4 and 250.9 ± 1.2 Ma for syenogranites, and 251.6 ± 1.4 Ma for porphyritic granites, representing a Late Permian magmatism record. These granitoids exhibit weakly peraluminous nature (A/CNK = 1.03–1.08) with high SiO2 (76.50–81.92 wt%) and K2O (3.64–5.56 wt%) concentrations and differentiation index values (D.I. = 93.8–97.2). Chondrite-normalized REE patterns of these samples display enrichment of LREE with high ratios of (La/Yb)N (12.29–30.34) and significant negative Eu anomalies (δEu = 0.15–0.53). In mantle-normalized multielement variation diagrams, they are all characterized by typical depletions of Ba, Nb, Ta, Sr, P, and Ti and enrichments of Rb, Th, U, and K. Petrological and geochemical features of the pluton indicate a highly fractionated I-type affinity. Zircon Hf isotope analyses show variably negative εHf(t) values ranging from −13.7 to −4.0 and corresponding two-stage model ages varying of 2148–1540 Ma, indicating that they were derived from an Paleo- to Mesoproterozoic crustal source with minor juvenile materials. In combination with previous geological evidences, it is reasonable to propose that the Sishijia granitoids emplaced in a post-collisional extension regime during the transformation from pre-collision to post-orogenic, which symbolize a Late Permian final closure of Paleo-Asian Ocean in southeastern segment of the Central Asian Orogenic Belt.

In situ major and trace element analysis of amphiboles in quartz monzodiorite porphyry from the Tonglvshan Cu–Fe (Au) deposit, Hubei Province, China: insights into magma evolution and related mineralization

The Tonglvshan deposit is the largest Cu–Fe (Au) skarn deposit in the Edong district, which is located in the westernmost part of the Middle and Lower Yangtze River metallogenic belt, China. In this study, we performed a detailed in situ analysis of major and trace elements in amphiboles from the ore-related Tonglvshan quartz monzodiorite porphyry using electron microprobe (EMPA) analysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Two distinct populations of amphiboles, which can be distinguished by their aluminum content, are found in the quartz monzodiorite porphyry. The low-aluminum (Low-Al) amphiboles are subhedral or anhedral and formed at 46.3–73.5 MPa and 713–763 °C. In contrast, the high-aluminum (High-Al) amphiboles are euhedral and formed at 88–165 MPa and 778–854 °C. Some euhedral amphiboles are partially or completely replaced by Low-Al amphibole. The compositions of parental melts in equilibrium with the High-Al amphibole (Melt 1) and Low-Al amphibole (Melt 2) were computed by applying solid/liquid partition coefficients. This modeling shows that magma in equilibrium with High-Al amphibole (Melt 1) underwent 40% fractional crystallization of amphibole, plagioclase and apatite at a depth of ~5 km to evolve to magma in equilibrium with Low-Al amphibole (Melt 2). Copper enrichment occurred in the magma after undergoing fractional crystallization. The magma had a high oxygen fugacity, increasing from NNO + 1 (Melt 1) through NNO + 2 to HM (Melt 2), which could have prevented the loss of Cu (and possibly Au) to sulfide minerals during crystallization. Finally, the evolved magma intruded to shallower depths, where it presumably exsolved aqueous ore-forming fluids. Therefore, the large Cu–Fe–Au reserves of the Tonglvshan deposit can likely be attributed to a combination of controlling factors, including high oxygen fugacity, fractional crystallization, fluid exsolution, and a shallow emplacement depth.

Genesis and petrology of Late Neoproterozoic pegmatites and aplites associated with the Taba metamorphic complex in southern Sinai, Egypt

We present new field, petrographical, mineralogical and geochemical data from late Neoproterozoic pegmatites and aplites in southern Sinai, Egypt, at the northernmost limit of the Arabian-Nubian Shield. The pegmatites cross-cut host rocks in the Taba Metamorphic Complex (TMC) with sharp contacts and are divided into massive and zoned pegmatites. Massive pegmatites are the most common and form veins, dykes and masses of variable dimensions; strikes range mainly from E-W through NW-SE to N-S. Mineralogically, the massive pegmatites are divided into K-feldspar-rich and albite-rich groups. Zoned pegmatites occur as lenses of variable dimensions, featuring a quartz core, an intermediate zone rich in K-feldspars and an outer finer-grained zone rich in albite. All compositions are highly evolved and display geochemical characteristics of post-collisional A-type granites: high SiO2, Na2O+K2O, FeO*/MgO, Ga/Al, Zr, Nb, Ga and Y alongside low CaO, MgO, Ba and Sr. They are rich in Rare Earth Elements (REE) and have extreme negative Eu anomalies (Eu/Eu*= 0.03–0.09). A genetic linkage between the pegmatites, aplites and alkali granite is confirmed by their common mild alkaline affinity and many other geochemical characteristics. These pegmatites and aplites represent the last small fraction of liquid remaining after extensive crystallization of granitic magma, injected along the foliation and into fractures of the host metamorphic rocks. The extensional tectonic regime and shallow depth of emplacement are consistent with a post-collisional environment.

Crystallization and uplift path of late Variscan granites evidenced by quartz chemistry and fluid inclusions: Example from the Land’s End granite, SW England

The megacrystic, coarse-grained granite of the Land’s End granitic complex, SW England, has been investigated by analyzing fluid inclusions, trace elements, and cathodoluminescence textures of quartz. By applying the TitaniQ geothermobarometer together with the cathodoluminescence textures, a two-stage emplacement process is proposed. K-feldspar and quartz phenocrysts crystallized in a deep magma chamber at ca. 18–20km depth. The phenocrysts were transported together with the melt to a shallow emplacement depth at ca. 5–9km in multiple intrusive events, causing the composite appearance of the granitic complex. This model of emplacement concurs with similar granites from the Erzgebirge. At the emplacement level, the magma exsolved an aqueous fluid with average salinity of 17.3% m/m NaCl and 9.7% m/m CaCl2. Fluids with higher salinities were exsolved deeper in the system, as the magma experienced stages of water saturation and water undersaturation during ascent from the deep magma chamber. The complex fluid inclusion textures are the results of multiple stages of entrapment of aqueous fluids in the host phases as multiple recharge events from the deeper magma chamber supplied fresh melts and aqueous volatiles. Titanium contents in quartz are closely related to the panchromatic cathodoluminescence intensity, and the Al/Ti ratio is reflected by the 3.26eV/2.70eV ratio of hyperspectral cathodoluminescence.

Asymmetric textural and structural patterns of a granitic body emplaced at shallow levels: The La Chinchilla pluton, northwestern Argentina

New mapping and a detailed structural study of the La Chinchilla stock, Sierra de Velasco, NW Argentina, suggest an asymmetrical shape of the pluton and provide strong evidence for its shallow emplacement depth. The pluton is a Lower Carboniferous monzogranite composed of K-feldspar, quartz, plagioclase and biotite. It exhibits an internal asymmetric textural zoning, defined by porphyritic granite in the southeastern region to equigranular granite in the northwestern region. The presence of subhorizontal dikes in the northwestern area, where the contacts dip shallowly, and subvertical dikes intruding the host rock nearby steep-dipping intrusive contacts in the southeastern region are compatible with an overall asymmetrical shape and internal structure of this pluton. Considering published crystallization ages, a dominant strain field occurring at around 12 Ma is inferred based on magmatic fabrics in the pluton and its host rock (the Huaco pluton), with a principal shortening direction oriented SW–NE, consistent with the general NW–SE strike of the body. Field evidence supports brittle fracturing and block displacement as the dominant emplacement mechanism, suggesting that magmatic stoping dominated during the late stage of the evolution of the magma chamber.

Late Ediacaran (605–580 Ma) post-collisional alkaline magmatism in the Arabian–Nubian Shield: A case study of Serbal ring-shaped intrusion, southern Sinai, Egypt

The Serbal pluton is a late Neoproterozoic (605–580Ma) post-collisional A-type granites in southern Sinai, Egypt (northernmost Arabian–Nubian Shield, ANS). It is characterized by discontinuous ring-shaped outcrops dislocated by later faulting. The pluton intrudes late Neoproterozoic metamorphic and high-K calc-alkaline rocks. The Serbal pluton mostly comprises an outer zone of alkali feldspar granite surrounding a core of peralkaline granite. Gradational and sharp contacts in the Serbal granites suggest that they were emplaced with a very short time interval, still before complete crystallization of the earlier batch. Serbal granites are highly evolved (75.98–78.52wt.% SiO2) and display the typical geochemical characteristics of post-collisional A-type granites, namely high SiO2, Na2O+K2O, FeO*/MgO, Ga/Al, Zr, Nb, Ga and Y and low CaO, MgO, Ba, and Sr. They are rich in REE and have extreme Eu-negative anomalies (Eu/Eu*=0.01–0.23). The chemical characteristics indicate that the peralkaline granite shares many features of granites with the tetrad REE effect. The Serbal pluton evolved through fractional crystallization of a parental magma derived through partial melting of a juvenile crustal protolith that had been extracted from a source having mantle geochemical and isotopic characteristics. The crystallization temperatures using Fe–Ti oxides of the Serbal granites point to their formation at high temperatures, up to 650–850°C at a shallow depth of emplacement (<10km).

The Cihai diabase in the Beishan region, NW China: Isotope geochronology, geochemistry and implications for Cornwall-style iron mineralization

Diabase dykes in Cihai, Beishan region, NW China are spatially and temporally associated with ‘Cornwall-type’ iron deposits. U–Pb dating of zircons from a diabase dyke using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields an age of 128.5±0.3Ma, indicating an Early Cretaceous crystallization age. Most of the diabases show low Mg-numbers, suggesting evolved magmas. The diabase dykes show typical ophitic or sub-ophitic textures, and are dominantly composed of phenocrysts of plagioclase (40–50%) and clinopyroxene (30–45%), with minor and varying amounts of biotite and hornblende (1–5%), and minor disseminated magnetite (∼5%). Their mineralogy reflects magma differentiation under relatively low oxygen fugacity conditions. The diabase dykes are characterized by minor variation in SiO2 (44.67–49.76wt.%) and MnO (0.14–0.26wt.%), but show a marked range of Al2O3 (10.66–14.21wt.%), total Fe2O3 (9.52–13.88wt.%), TiO2 (0.66–2.82wt.%) and relatively high MgO (4.87–9.29wt.%) with an Mg# value [atomic Mg/(Mg+Fe2+)] of up to 66. The Cihai diabases possibly experienced fractional crystallization of olivine+clinopyroxene and minor crustal contamination during the differentiation process. Prominent negative Nb, Ta and Ti anomalies suggest derivation from subduction-modified mantle. Furthermore, the rocks have relatively unradiogenic Sr- and Nd-isotopic ratios. These characteristics probably reflect partial melting of a subduction component in the source mantle lithosphere through heat input from an upwelling asthenospheric mantle. Such processes probably occurred within an extensional setting during the Early Cretaceous in the Beishan area. The iron-rich fluids were derived from deep sources, and the iron ores were concentrated through a convection cell driven by temperature gradients established by the intrusion of the diabase sills. The combined processes of subduction-related enrichment in the source, shallow depth of emplacement, and the involvement of large-scale circulation of basinal brines from an evaporitic source are inferred to have contributed to the formation of the ‘Cornwall-type’ mineralization in Cihai.

Al-in-Hornblende Thermobarometry and Sr-Nd-O-Pb Isotopic Compositions of the Early Miocene Alaçam Granite in NW Anatolia (Turkey)

During and after the closure of the Neo-Tethyan Ocean and progressive collision of the Tauride-Anatolide Platform with the Sakarya Continent, widespread magmatism occurred in NW Anatolia. This magmatism is manifested in a NW-trending belt along the northern border of the Menderes Massif. Due to the complex geodynamic setting of this region, the exact emplacement depth of the granitoids is still a matter of debate. Here we present Al-in-hornblende barometrical data and Sr-Nd-Pb isotope compositions of the Early Miocene Alaçam granite. The results imply a shallow emplacement depth of this granite (4.7±1.6 km) in contrast to previous studies which suggested emplacement along the brittle-ductile boundary of the crust. Furthermore, an evaluation of literature data let us reconsider the general emplacement mechanism of the Alaçam and other Early Miocene granitoids in the region. Initial isotopic signatures of the Alaçam granite are 87Sr/86Sr(I)= 0.70865-0.70915, eNd(I)= -5.8 to -6.4, \delta^1^8O= 9.5-10.5, 206Pb/204Pb isotope ratios vary between 18.87 and 18.90. These features indicate an assimilation-dominated crustal crystallization and melt derivation from an older middle crustal protolith.

Character and significance of spectacular layering features developed in the thin, alkali-basaltic sills of the Ulvö Gabbro Complex, Sweden

The Ulvo Gabbro Complex consists of alkali-olivine basaltic circular bodies ∼30–80 km in diameter. These intrusions were emplaced at shallow depths (∼3 km) as thin sheets (∼300 m). Among other things, the gabbroic cumulates of the complex display: modal layering, grain-size variations, trough structures, and slump structures. The crystallization sequence is olivine+plagioclase, ulvospinel, clinopyroxene, and apatite. A nearly continuous exposure across one of these intrusions, the Norra Ulvon gabbro, is subdivided into: a Lower Zone (LZ), a Rhythmically Layered Zone (RZ) and an Upper Zone (UZ). LZ and RZ were formed at the floor, while UZ grew from the roof downward. Major-element variations in the cores of the cumulus minerals define fractionation trends from the base of the intrusion to the RZ-UZ boundary interpreted as a “sandwich horizon”. Modeling suggests that a significant amount of crystallized interstitial liquid is required to produce the observed stratigraphic relations. Our results suggest that the small size and shallow emplacement depth of the intrusions of the Ulvo Gabbro Complex helped to preserve evidence of primary accumulation processes. However, it is also clear that despite the limited time available postcumulus processes such as diffusional homogenization and compaction of some grains were important.

A record of crustal-scale stress; igneous foliation and lineation in the Mount Waldo Pluton, Waldo County, Maine

The Late Devonian Mount Waldo pluton is a porphyritic granite that has a magmatic foliation and lineation. Foliation is defined by: (1) preferred orientation of potassium feldspar phenocrysts and biotite crystals, and (2) the anisotropy of magnetic susceptibility (AMS). AMS alone defines the lineation. The carrier of the AMS signal is multidomain magnetite, confirmed by petrographic observation, high bulk susceptibility (generally >3 E3 SI units) of samples, and results of isothermal remanent magnetization experiments. AMS data from 26 sites in the pluton consistently show a subhorizontal north–south-trending lineation (defined by the orientation of the maximum axis of the AMS ellipsoid) and a subvertical north–south-striking foliation (whose pole is the minimum axis of the AMS ellipsoid). At individual sites, the feldspar and AMS foliations are parallel. The lack of evidence for significant solid state deformation in the pluton suggests that both the lineation and foliation are entirely magmatic in origin. The pluton's relatively small size (15 km diameter) and shallow emplacement depth (≤3 kbar) suggest that it was in its magmatic state for a short time, probably less than one million years. The orientation of the lineation and foliation is consistent with their formation due to a small increment of crustal-scale strain absorbed by the pluton in its geologically brief magmatic state. If so, these fabrics record a Late Devonian crustal-scale strike-slip stress regime that may also have driven dextral movement on the neighboring Norumbega Fault Zone.

Uijongbu circular structure of Seoul granitic batholith, Korea: ring dike origin of a Jurassic volcanic cauldron

We report a ring dike of Jurassic granite for the first time in Korea. There is a prominent circular structure in Seoul granitic batholith near Uijongbu, north of Seoul. The circular structure consists of large eastern half-circle (ca. 13 km in diameter) and small western half-circle (ca. 6 km in diameter) divided by a north-south-striking fault. A comparison with recently available geologic maps indicates that the geologic structure corresponds to a ring dike (or cone sheet) of garnet biotite granite, which negates an impact origin. We suggest that the Uijongbu structure was formed by differential weathering between resistant garnet biotite granite and easily weathered biotite granite. A ring dike origin for the Uijongbu structure probably associated with a volcanic cauldron subsidence is supported by rather shallow emplacement depth for the Jurassic granite and fast cooling rate (ca. 100°C/Ma) estimated from mineral ages, although no associated volcanics are present in the surrounding region.

Depth of emplacement and tilting of the Middle Proterozoic (1470 Ma) Wolf River batholith, Wisconsin: 40Ar/39Ar thermochronologic constraints

The 1470 Ma Wolf River batholith of northeastern Wisconsin is one of the oldest and largest intrusive sequences of the Middle Proterozoic magmatic suite that trends across the continent from Labrador to southern California. The presence of miarolitic cavities in portions of the batholith and in associated plutons (Wausau syenite complex) and equilibrium considerations suggest a shallow emplacement depth (i.e., &lt;4 km). However, these data contrast with published Rb/Sr biotite ages, which suggest much of the batholith was uplifted through mid-crustal temperatures well after intrusion. To address this problem better, we have obtained four biotite 40Ar/39Ar dates from a transect across the southwestern portion of the batholith, one date from the ~1520 Ma Wausau syenite complex, and another from rocks metamorphosed during the Early Proterozoic Penokean orogeny. The two oldest dates are concordant (within error) with the crystallization age of the batholith, suggesting rapid cooling after intrusion and hence shallow emplacement depth. Cooling dates young to the south-southeast, however, and the youngest dates within the batholith (~1400 Ma) are concordant with previous Rb/Sr biotite dates and with the Ar/Ar biotite date from the adjacent country rock. The data suggest slow cooling and southeast-side-up tilting of this portion of the exposed batholith from mid-crustal temperatures (~300°C). Postemplacement uplift and tilting of the batholith appear related to differential isostatic uplift driven by substantial original thickness variations of the intrusion.

A comparison between a downhole seismometer and a seismometer on the ocean floor

Abstract We have compared the signal and noise characteristics recorded by a downhole seismometer and a nearby (within 200 m) ocean bottom seismometer in the deep ocean (5.5 km). Absolute noise levels and signal-to-noise ratios from the two instruments are remarkably similar. We attribute this unexpected similarity to the shallow emplacement depth of the downhole instrument (194 m subbottom) and the soft sediment into which it was clamped.

The Mesozoic La Mula Island, Coahuila, Mexico

Granitic rocks at Potrero de La Mula and Sierra del Fuste, Coahuila, Mexico, are the only known exposures of a Late Triassic pluton that, by the Late Jurassic, stood as a topographically high area until it was inundated by the Cretaceous sea. The principal igneous rock at Potrero de La Mula is a two-feldspar, biotite-hornblende, I-type granite that is cut by comagmatic dikes of six ages and by several shear zones. A Rb-Sr age of 213 ± 14 m.y. and an initial ratio of 0.705 ± .0004 were obtained from nine whole-rock samples. A shallow depth of emplacement is inferred from textural and mineralogical evidence. Mineralogy, chemistry, and isotopes suggest emplacement above a subduction zone, which is consistent with the hypothesis that a late Paleozoic-earliest Mesozoic convergent plate boundary existed in northern Mexico. After being unroofed, the pluton contributed sediment to the adjoining Sabinas Gulf until the mid-Neocomian. By the Hauterivian, the pluton supported, a deeply weathered terrain of moderate relief flanked on its northeastern side by lagoonal environments (Oballos Member, Padilla Formation). Farther south, uplift by faulting along the northern flank of the Coahuila Island produced a clastic wedge, the finer-grained paralic components of which (La Mula Formation) prograded northeastward and eventually covered La Mula Island. Subsequent deposits at Potrero de La Mula, beginning with gypsum, reflect gradually deepening water and include the La Virgen, Cupido, La Pena, and Tamaulipas Formations.

Petrology and regional significance of the Roxboro Metagranite, North Carolina

The pluton located at Roxboro, North Carolina, is predominantly a light gray to medium gray, microphaneritic metagranite. Phenocrysts of plagioclase, quartz, and perthite are accompanied by porphyroblasts of epidote. Relict igneous textural relationships suggest two possible fractional crystallization models, in both of which the order of crystallization was plagioclase, quartz, and then K-rich feldspar. A crude approximation of the composition of the original plagioclase phenocrysts is An 25 to An 40 . Based on the composition of locally present granophyre, the pluton was emplaced under almost dry conditions with a P total of about 350 bars and a temperature in the vicinity of 950 °C. The shallow depth of emplacement suggests that the Roxboro Metagranite represents the root of a volcanic sequence that once unconformably overlay the presently exposed sequence. During the middle Paleozoic, this granitic intrusive was metamorphosed at a minimum pressure of about 3 kb and a temperature of approximately 400 °C. A foliation, as shown by stringers of mainly biotite and epidote, was produced by the deformational phase accompanying regional metamorphism. All K-rich feldspar is now nearly pure microcline (∼Or 97 ), and all plagioclase is now nearly pure low albite (Ab 97–99 ). Such feldspar compositions accompanied by late growth of ferristilpnomelane indicate a re-equilibration under lower grade conditions than those realized during the peak of the major regional metamorphic event.