Crustal Residence Times Research Articles

Precise lead isotope fingerprinting of hydrothermal activity associated with Ordovician to Carboniferous metallogenic events in the Lachlan fold belt of New South Wales

The Pb isotope signatures of sulfide mineralization within the Lachlan fold belt can be divided into those containing almost exclusively mantle-derived Pb, those containing Pb which has had a long crustal residence time, and those containing Pb of mixed mantle and crust parentage. In the Early Paleozoic (Ordovician to Silurian) all the major and the majority of other deposits have either crust or mantle signatures with very little evidence of mixing. Porphyry to epithermal Cu and/or Au mineralization hosted within Ordovician shoshonitic rocks (e.g., the Goonumbla deposits) have a wide range of 206 Pb/ 204 Pb ratios (17.68-18.21) and very low 207 Pb/ 204 Pb (15.40-15.49) and 208 Pb/ 204 Pb (37.21-37.83) ratios. Ores and their corresponding host rocks have very similar isotopic compositions suggesting that Pb is magmatic in origin On 208 Pb/ 204 Pb vs. 206 Pb? 204 Pb and 207 Pb/ 204 Pb vs. 208 Pb/ 204 Pb diagrams, these data plot on very precise lines (mantle mixing lines) which represent mixing of Pb from two or more mantle reservoirs. It is postulated that the position of each deposit along this line is an indication of the timing of metallogenesis within the Ordovician magmatic cycle and possibly also the degree of mixing of melts derived from a more primitive asthenosphere and a more enriched lithosphere, potentially fertile for Cu and Au. Exploration samples which plot on the mantle mixing lines and have high 206 Pb/ 204 Pb ratios are considered to have the best chance of representing a large metallogenic event. Deposits which are spatially related to Ordovician volcanics but which have more crustlike Pb isotope signatures and are generally relatively small in size, are considered to have formed in response to younger (Silurian? to Devonian) magmatic or metamorphic events which have recycled Pb from the Ordovician rocks and mixed it with crustal Pb...

Petrogenesis of a Mid-Proterozoic Anorthosite-Mangerite-Charnockite-Granite (AMCG) Complex: Isotopic and Chemical Evidence from the Nain Plutonic Suite

The Nain Plutonic Suite (NPS), about 1.30 Ga, underlies some 19,000 km², intruding the boundary between Middle to Late Archean rocks in the east (Nain Province) and Late Archean and Early Proterozoic rocks in the west (Churchill Province). Nd isotopic compositions in most igneous units of the NPS reflect their geographic position relative to the inferred boundary between the Nain and Churchill Provinces. Rare exceptions are local intrusions of olivine-bearing basic rocks with highest $$\epsilon_{Nd}$$ @ 1.3 Ga (near -3.0), present in both eastern and western sectors of the NPS. Anorthositic rocks have variable and significant contributions of crustal Nd but notably lower $$I_{Sr}$$ than granitoid rocks. Ferrodiorites are similar isotopically to the anorthositic group and have chemical and mineral compositions more akin to the anorthositic group than the granitoids. Aluminous orthopyroxene megacrysts in NPS anorthositic rocks indicate crystallization pressures of 6.2 to 11.0 kbar, consistent with lower crustal to upper mantle depths. The granitoid rocks crystallized from magmas derived in large part by crustal partial melting. These rocks show the influence of sources similar to the basement gneisses, but were augmented by materials with much shorter crustal residence time, offering support for contemporaneous basaltic underplating of the lower crust. Nd and Sr isotope data, trace element geochemistry, and mineral compositions in major rock units of the NPS, together with knowledge of the timing of intrusive events, permits assessment of the essential features of petrogenesis and development of a paradigm that may have wider application. Separation of early granitoid melts left depleted, hot, plagioclase-pyroxene granulite crustal residues assimilated by mantle-derived basaltic magmas. That process developed large volumes of anorthositic magma in upper levels of deep crustal to uppermost mantle magma chambers by plagioclase flotation. Concurrently, fractionation moved residual liquids toward ferrodiorite compositions. Buoyant upward movement of anorthositic magmas followed crustal paths preheated by earlier passage of granitoid magmas. Only small volumes of high density, Fe-rich, ferrodiorite melts were expelled to higher crustal levels.

Off-axis volcanism at the East Pacific Rise detected by uranium-series dating of basalts

RECENT detailed surveys of the East Pacific Rise have revealed the complexity of the volcanic and magmatic processes occurring along and across fast-spreading ocean ridge crests1–7. In parallel with geological and geochemical investigations, it is now possible to investigate the temporal and spatial pattern of volcanism at ocean ridges by dating young basalts using mass spectrometric uranium-series disequilibria methods8–11. Here we use 238U-230Th and235U-231Pa ages for basalts to quantify the spatial extent of young volcanism and crustal accretion at 9°31′ N on the East Pacific Rise. Most of the ages are younger than would be expected based on off-axis distance and spreading rate. We infer from these anomalously young ages that most of the dated basalts on the crestal plateau were erupted 0.5–2 km outside the axial summit caldera, with some volcanism occurring as far as 4 km off-axis. Melts erupted outside the axial summit caldera can have crustal residence times and magmatic supply systems that differ from those of axial lavas.

The Al Ays Batholith: A Late Proterozoic Granitoid Batholith Associated with the Al Ays Volcanic Arc Complex, Hijaz Terrane, Arabian Shield

The Al Ays batholith is located in the Hijaz microplate of the Arabian Shield. It intrudes the volcanic facies (andesitic and basaltic lavas and pyroclastic material) of the mature arc-derived Al Ays volcanosedimentary sequence. The batholith is composite, 3-30 km wide by about 50 km long, and ranges from diorite to granite. A 5-point Rb/Sr isochron gave an ageo f706:t 2 Maand an initial 87SrJ86rSarti oofO. 70319:t 0.00002. The granitoid rocks of the batholith are calc-alkaline, metaluminous to subaluminous, with moderate LREE enrichment and a negative Eu anomaly, and show a wide compositional range (SiO254.40-72.30%, Cao 1.16- 6.29%, K20 1.8-3.94%, Rb 7-85 ppm. Sr 89-397 ppm, Y 7-40 ppm, Zr 40- 337 ppm, Rb/Sr ratios 0.06-0.95). The young age, calc-alkaline association, compositionally expanded nature with a spectrum of rock types from dioritic to granitic, short crustal residence time, and the syn- to late-collisional tectonic 'environment all suggest a model that the Al Aysvolcano-plutonic complex (743 :t 12.to 725 :t 12 Ma) formed as a result of subduction and melting of oceanic lithosphere beneath the pre-existing arc-basement of volcano-plutonic rocks and their sedimentary derivatives.

Lead isotopic evidence for mixed sources of Proterozoic granites and pegmatites, Black Hills, South Dakota, USA

The lead isotopic compositions of K-feldspars separated from the ca. 1700 Ma Harney Peak Granite complex and spatially associated granitic pegmatites indicate that these rocks were derived from at least two sources. It has been reported previously that the core of the Harney Peak Granite complex is dominated by relatively lower/ gd 18O (avg. 11.5 %.) granites, whereas higher / gd 18O (avg. 13.2%.) granites occur around the periphery of the complex. The higher δ 18O granites and one simple pegmatite have low values of 207Pb 204Pb for their 206Pb 204Pb Thus, they likely were derived from a source with a short crustal residence time. This source may have been the pelitic schists into which the Harney Peak Granite complex and pegmatites were intruded. Feldspars from granites with lower / gd 18O values have significantly higher 207Pb 204Pb for their 206Pb 204Pb . The data define a linear array with a slope equivalent to an age of ca. 2.6 Ga with t 2 defined to be 1.7 Ga. Such a slope could represent a mixing array or a secondary isochron for the source. These low δ 18 O granites could have been derived from a source with a high U/ Pb and with a crustal residence beginning before the Proterozoic. The source (s) of these granites may have been a sediment derived from late Archean continental crust. The highly evolved Tin Mountain pegmatite has lead isotopic systematics intermediate between those of the two granite groups, suggesting either a mixed source or contamination. Two late Archean granites, the Little Elk Granite and the Bear Mountain Granite, had precursors with high U Pb and low Th U histories. The Th U history of the Bear Mountain Granite is too low for this rock to have been an important component of the source of the Proterozoic granites. However, crustal rocks with lead isotopic compositions similar to those of the Little Elk Granite were an important source of lead for some of the Proterozoic granitic rocks.

Reconnaissance isotopic geochemistry of anorthosite mangerite-charnockite-granite (AMCG) complexes, Grenville Province, Canada

Anorthosite-mangerite-charnockite-granite (AMCG) complexes with ages of 1.13–1.16 Ga constitute the most widespread and voluminous igneous suites in the Grenville Province. The complexes are widely believed to comprise mantle-derived anorthosite and related basic rocks together with mangerites, charnockites and granites of largely crustal origin. This paper presents results of geochemical and Nd, Sr and Pb isotopic studies of basic and silicic samples from five AMCG complexes in the Grenville Province. Anorthositic members all have positive ϵ Nd (+0.8 to +4.5) indicating time-integrated incompatible element depleted sources, in agreement with earlier studies. Most granitoid samples, with one notable exception, also have positive ϵ Nd (+1.6 to +3.4) indicating sources with short crustal residence times. Granitoids from the Atikonak River complex in southern Labrador have ϵ Nd −3.7 to −2.2 revealing clear evidence that partial melting of relatively old crust was involved in their formation. For both anorthositic and granitoid samples there is broad correlation of decreasing ϵ Nd with increasing Sr i imparting confidence in a model requiring at least two fundamentally different end-members. Isotopic studies of these Grenville Province AMCG complexes, excluding Atikonak, are in accord with a two-source origin in which the crustal sources were not more than a few hundred Ma old at the time they were partially melted; however, a single source origin is not excluded for complexes like Mealy Mountains that evidently developed in newly-formed crust. The petrology and geochemistry of AMCG complexes is broadly consistent with processes whereby crustal partial melting at depth is driven by proximity to basic magma. Extraction of granitoid partial melts is expected to leave crustal residues of appropriate composition to produce anorthositic magmas when assimilated by subjacent basic magmas.

Petrogenetic implications of neodymium isotope data from the Kohistan batholith, North Pakistan

Neodymium data are presented for five granitoid (trondhjemite, granite and leucogranite) plutonic units from the Kohistan batholith aged 102 Ma to 29 Ma. These have present day l43 Nd/ 144 Nd ratios of between 0.512980 and 0.512734, calculated initial 143 Nd/ 144 Nd ratios of between 0.512861 and 0.512705 and Nd values of between 6.91 and 2.04. There is a decrease in ɛNd with time which is inversely correlative with a similar increase in ɛSr. Three plutonic units (Matum Das, Gilgit and Shirot) formed from a source enriched in Sm and depleted in Rb or radiogenic Sr relative to bulk earth, whilst a fourth unit (the Indus Confluence acid sheets), is only slightly enriched in radiogenic Sr. These four units define a temporal trend from the least evolved Matum Das pluton to the most evolved Indus Confluence acid sheets. This trend was produced as a result of subducting oceanic sediment or seawater-altered oceanic crust which melted or dehydrated and increasingly modified the isotopic composition of an original mantle protolith situated above the subducting plate. The data preclude any significant input to the magmatism from ancient crust. New data presented here, together with other published data, indicate an immature metavolcanic crustal source for the Matum Das pluton, and a plutonic basement crustal source for the Indus Confluence acid sheets. These crustally derived units retain a mantle isotopic signature indicating that the geochemical signature of Kohistan evolved by remobilization of recently formed arc crust in addition to new inputs of mantle-derived magma. A fifth plutonic unit (the Parri acid sheets) shows a clear compositional break from the other units, being significantly enriched in radiogenic Sr and Nd with respect to bulk earth. The source to the Parri acid sheets is interpreted as metasedimentary rocks with a high Rb/Sr ratio and a crustal residence time of c. 70 Ma.

Crustal growth in West Africa at 2.1 Ga

Birimian (∼2.1 Ga) terranes in the West African craton are a mixture of highly metamorphosed volcanic, sedimentary and plutonic rocks and low grade metavolcanics and metasediments. The volcanic units contain thick, commonly pillowed, tholeiitic basalts overlain by pelagic sediments cherts and black shales; The sedimentary units are characterized by an abundance of clastic turbiditic sediments. Andesites and calc‐alkaline felsic volcanics occur at uppermost stratigraphic levels and as dykes. Field relationships between the volcanic and sedimentary units remain a matter of debate. Calc‐alkaline and local alkaline granites, which intruded in distinct pulses and occasionally are related to transcurrent tectonics, represent almost half of the Birimian terranes. New isotopic work on the highly metamorphosed units greatly improved the chronology for the Birimian crust. The age of the early Dabakalian event is precisely defined by a U‐Pb zircon age at 2186 ± 19 Ma, while Rb‐Sr and Sm‐Nd methods give ages of 2162 ± 19 Ma and 2141 ± 24 Ma, respectively. A Sm‐Nd garnet‐whole rock age of 2153 ± 13 Ma suggests that metamorphism culminated at about the same time. In contrast, the most precise zircon U‐Pb and Sm‐Nd data for the more widespread Birimian terranes (sensu stricto), from this study and from the literature, cluster between 2.12 and 2.07 Ga. The major evolution of the Birimian crust apparently lasted less than 50 Ma. Isotopic evidence indicates that Birimian granitoids contain a negligible component of Archean crust: εNd(2.1‐Ga) values are positive and similar to those of Birimian basalts, crustal residence times are shorter than 200 Ma, U‐Pb ages for detrital zircons from clastic sediments range from 2098 ±11 Ma to 2125 ± 17 Ma, while granite chemistry and Nd isotopic characteristics are unrelated. Only very locally in Guinea is there isotopic evidence of interaction between Birimian felsic magmas and the Archean rocks from the Man craton. In accord with Abouchami et al.'s (1990) suggestion that Birimian basalts represent oceanic plateaus, the present data argue that the protolith of much of the West African continent was created around 2.1 Ga in an environment remote from Archean crust. Intrusion of calc‐alkaline magmas into the tholeiitic units suggests that island arcs formed on top of the assumed oceanic plateaus which then collided with the Man Archean craton. Taking the Birimian formations from the Guyana shield into account, the minimum crustal growth rate at 2.1 Ga is about 1.6 km3/a, some ∼60% higher than the present growth rate. Birimian crust growth at 2.1 Ga is reminiscent of Archean processes but contrasts with 1.7 – 1.9 Ga crust formation in the North Atlantic continent which generally involved significantly more interaction with older continental crust. A comparison of the Birimian crustal growth rate with the average crustal growth rate over the Earth history implies that a large part of the Birimian crust has been recycled into the mantle or incorporated into younger orogenic segments. This apparent deficit in the crustal budget is even more dramatic for the Archean crust.

The Great Whinscale Dacite — an enigmatic lava flow from the Borrowdale Volcanic Group, English Lake District

SUMMARY The Great Whinscale Dacite (GWD) is a distinctive and important stratigraphic marker unit within the Ordovician Borrowdale Volcanic Group (BVG) of the English Lake District. Many features of the GWD are compatible with the unit being a lava — the origin proposed here — but with a low aspect ratio ( c .1: 100), a length of at least 13 km, and a minimum volume of 4 km 3 . The GWD is aphyric, remarkably homogeneous chemically, and is highly atypical of dacitic lavas in the BVG, which are distinctly porphyritic and of very restricted extent and volume. This paper presents field, petrographic, chemical and Rb-Sr isotopic age data from four transects through the GWD and one transect through a typical BVG dacite at Crowhow End for comparison. Sm-Nd isotope data are presented for selected samples. Although the overall chemistry of the GWD is very similar to other BVG dacites, the GWD has a distinctly more primitive Nd isotopic signature and is noticeably depleted in Ni and Cr. The aphyric nature of the GWD and its distinct chemical character may be attributed to an abnormally short crustal residence time for the GWD magma between source and eruption, and thus emplacement at a temperature above the liquidus. The implied short rise-time also resulted in a high effusion rate which controlled the unusual eruption geometry. Subsequently there were at least three distinct periods of isotopic and chemical disturbance.

Geology, geochemistry, and isotopic character of the Colville Igneous Complex, northeastern Washington

The Colville Igneous Complex in northeastern Washington comprises Paleocene orthogneisses and leucocratic granitic rocks and Eocene volcanic‐plutonic suites which were emplaced during early Tertiary extension. The Colville Igneous Complex is divided into three components: (1) 65 ± 4 Ma orthogneisses of the Okanogan dome or metamorphic complex and similar gneisses in the Kettle and Lincoln complexes; (2) 61–51 Ma leucocratic granitic rocks; and (3) 53–47 Ma intermediate to felsic volcanic‐plutonic rocks. The leucocratic granites and younger volcanic‐plutonic rocks are divided on the basis of age, geochemical and isotopic characteristics, and emplacement mechanism. The older leucocratic granites are silica rich, corundum‐normative, and Sr and light rare earth element (REE)‐enriched, and they form several large, discrete, compositionally zoned plutons and intrusive suites. The younger, Eocene volcanic‐plutonic rocks can be divided into two suites: an older compositionally restricted (68–72wt % SiO2), Sr‐enriched, medium‐K suite and a younger intermediate to felsic, high‐K, calcalkalic suite. Sr isotopic measurements of volcanic and plutonic rock in the Colville Igneous Complex yield variable initial 87Sr/86Sr ratios (Sri), ranging from about 0.704 to values greater than 0.710. In general, the volcanic rocks are especially variable with little or no correlation between Sri and geographic location. The leucocratic granites, however, show a systematic west to east increase across the area which probably reflects an increased involvement with Precambrian crust. The variable nature of Sri in the Colville Igneous Complex may also be controlled by crustal thickness and/or depth of melting. Within individual volcanic and plutonic suites, there is a crude correlation of mean Sri with age. There is a systematic decrease in Sri with increasing age which may reflect either a migration of the site(s) of melting or a decreased upper crustal residence time, possibly related to the onset of extension. The leucocratic granites may have formed by small degrees of partial melting in the lower crust, perhaps facilitated by thermal blanketing of an initially overthickened crust. The ascent of large bodies of leucocratic magmas may have increased the geothermal gradient which allowed for upper, more radiogenic, crust to be melted.

Late proterozoic fractionated granitoids of the mineralized Telfer area, Paterson province, Western Australia

Abstract The Proterozoic Paterson Province is a newly defined, polymetallic province which has been interpreted to represent a collisional orogenic belt between the geologically contrasting Western Australian Craton and Northern Australian Province. It contrasts with the Proterozoic terrains of Northern Australia which are interpreted to be intracratonic basins. Within the Telfer area of the Paterson Province, late- to post-tectonic monzogranites to alkali-feldspar granites intrude the upper Yeneena Group (> 750, The granitoids are highly fractionated (average SiO2 content > 71 wt.%, Rb/Sr ranges up to 20.5), metaluminous and I-type in nature, and some suites have high contents of heat-producing elements (average pre-weathering U and Th contents of 15 ppm and 66 ppm, respectively, and average K2O content of 4.9 wt.%). On a global scale these features are common to granitoids associated with mineralization. The granitoids generally show geochemical coherence on variation diagrams, but the contrasting Pb-isotope initial ratios for two of the complexes indicate that they are not cogenetic. This limited geochemical variation is typical for fractionated granitoids, due to a minimal restite component, and the behaviour of trace elements is controlled by the fractional crystallization of accessory minerals. Similarly, the granitoids plot close to boundaries between fields for collisional, volcanic-arc and within-plate granitoids because of their highly fractionated nature (i.e. high Rb): this is probably a general problem for fractionated granitoids on such discriminant diagrams. However, the Telfer granitoids can be broadly discriminated as syn- to post-collisional with calc-alkaline compositions. The granitoids of the Telfer area are thus late- to post-tectonic granites whose age and geochemistry are both significantly different to other Proterozoic granites in Northern Australia. Their young age, calc-alkaline I-type association, the short crustal residence time of their source and their interpreted syn- to post-collisional environment all support the recently proposed model that the Paterson Province represents a unique tectonic setting in the Proterozoic, involving the welding of two continental plates at ∼690 Ma.

Radial growth rates and 210Pb ages of hydrothermal massive sulfides from the Juan de Fuca Ridge

Activities of 238U, 228Ra, 226Ra and 210Pb were determined in submarine hydrothermal massive sulfides by nondestructive, gamma-ray spectrometry. The samples were collected by the manned submersible DSRV “Alvin” from active hydrothermal fields on the Endeavour Segment and Axial Seamount of the Juan de Fuca Ridge. 210Pb activities are mostly below the equilibrium level with 226Ra, which is linearly correlated with the concentration of Ba, Sr and Ca in the deposits. The ratios of 226Ra to the alkaline earth elements indicate that hydrothermal alteration of the underlying oceanic crust is the dominant source of Ra in the sulfide deposits. The 210Pb/Pb ratios measured in many of the sulfides are higher than the ratio obtained for a basalt source of 210Pb, probably because of 210Pb ingrowth from 226Ra within the sulfide deposits. An isochron approach employing ratios of 210Pb/Pb and 226Ra/Pb yielded an initial 210Pb/Pb ratio in the range of 0–0.57 dpm/μg Pb, implying that the crustal residence time of the hydrothermal fluid in the Endeavour Segment is very short ( < 10 years) and that basalt alteration is the only source of the 210Pb and Pb. For the hydrothermal fluid in Axial Seamount, a residence time of 10–20 years is estimated on the basis of the initial 210Pb/Pb and 228Ra/ 226Ra ratios of a sulfide chimney. Both residence time estimates are consistent with the results of previous studies. We have estimated the upper limits of sample ages ranging from tens to a hundred years for 12 samples, with the remaining seven samples indicating ages close to or older than 150 years, the practical limit of the 210Pb/Pb dating technique. A concentrically sectioned sulfide chimney from Axial Seamount yielded mean radial growth rates ranging from a few tenths to a few mm/year based on gradients of 210Pb/Pb and 228Ra/ 226Ra activity ratios, whereas a similarly sectioned chimney from the Endeavour Segment, although collected from the top of an active “black smoker” vent, displayed no 210Pb or 228Ra activity. Our results have implications for the duration and periodicity of hydrothermal circulation and associated mineral deposition at mid-ocean ridges.

The Late Archaean to Early Proterozoic lead isotopic evolution of the Northern Baltic Shield of Norway, Sweden and Finland

The Late Archaean rocks of the Northern Baltic Shield comprise high-grade gneiss terrains and granite-greenstone terrains. The high-grade gneiss terrains consist mainly of older upper-crustal material which became metamorphosed during the Lopian orogeny (ca. 2.9–2.6 Ga). In contrast, the ∼2.7 Ga-old granite-greenstone terrains comprise mainly mantle-derived materials, At the southern and western edge, the Archaean craton became partially remobilized during the Svecofennian orogeny (2.0–1.75 Ga). This orogeny represents a major crust-forming event in the Baltic Shield, during which mantle material was added to the crust and older material was remobilized. Whole-rock, galena and sulfide trace lead are used to outline lead provinces on the Northern Baltic Shield. The Archaean lead from the Baltic Shield defines three secondary lead isochrons at ∼ 2.65 Ga with different initial lead compositions which reflect average pre-2.65 Ga μ 1 ratios of 8.0, 8.1 and 8.3, respectively. The variation of the initial lead composition of the high-grade gneisses and tonalites (Lofoten and Koitelainen area) probably indicates (1) variable crustal residence time and/or (2) different degrees of crustal contamination/assimilation before the 2.65 Ga metamorphic event, and the lead from the Eastern Finland greenstone belts indicates a heterogeneous mantle with respect to U/Pb before 2.65 Ga. Lead in Proterozoic rocks of the different lead provinces defines sets of subparallel mixing fields which can be modelled by a ≈2.65 Ga lead component and a ∼ 1.8 Ga lead component. The parallel offset of these fields reflects the pre-2.65 Ga μ 1 ratio of these terrains, i.e. it reveals the pre-2.65 Ga μ 1 of the involved Archaean rocks. Proterozoic terrains with involvement of Archaean lower-crustal lead show a poor correlation in the 206Pb/ 204Pb- 208Pb/ 204Pb diagram, whereas the other Proterozoic terrains show a good correlation in the 206Pb/ 204Pb- 208Pb/ 204Pb diagram.

Sm–Nd and trace-element characterization of shales from the Abitibi Belt, Labrador Trough, and Appalachian Belt: consequences for crustal evolution through time

Nd-isotopic compositions and Sm, Nd, Li, K, Rb, Sr, Ba, Ni, and Cr abundances are reported for 25 shale samples from the Canadian Shield (late Archean Abitibi greenstone belt and the mid-Proterozoic Labrador Trough) and from the Quebec Appalachians (lower Paleozoic Humber Zone). The chemical and isotopic characteristics of the samples are used to monitor the rate of generation and the compositional evolution of continental crust through time. The Nd crustal-residence ages record preferential time of continental growth around 2.7 and 1.7 Ga. The Nd model ages of the Appalachian shales do not record evidence for the formation of large crustal volumes through mantle extraction since 1.3 Ga. Consequently, crustal recycling was the dominant process taking place at their source areas in the Grenville Province.The trace-element distributions of shales show systematic trends as a function of time: Li, K, Ba, Sm, and Nd contents regularly increase in the post-Archean record; in comparison, the Cr and Ni contents reached a maximum towards the end of the Archean and regularly decreased thereafter. These observations could reflect two classes of processes: (a) the development of infracrustal K-rich granitoid magmatism at the expense of mantle-derived Na-rich magmatism, which dominated the Archean period; or (b) differential erosion effects, which reduced the sampling of the old, smooth crustal parts in comparison to the younger, recycled segments. In the latter case, (b), shale formation involved components whose nature and respective proportion changed after Archean time.

Isotopic evidence for crust-mantle evolution with emphasis on the Canadian Shield

Nd, Sr and Pb isotope data from carbonatites and related silicate rocks spanning an age range from 0.1 to 2.7 Ga in the Grenville and Superior provinces, Canadian Shield, trace the secular geochemical evolution of large ion lithophile element-depleted mantle sources beneath those provinces over the past 2.2 Ga. Sr data from 2.67-Ga-old syenites from the Abitibi, Quetico and Wabigoon belts in the Superior Province do not indicate depleted mantle sources, although ϵ Nd for the same rocks averages +1. Initial Pb ratios in the syenites yield single-stage model ages that agree closely with their radiometric ages, while the model ages for the younger complexes are substantially younger than their radiometric ages. This suggests either that Pb in the 2.67-Ga complexes was not derived from aged depleted mantle sources, or that any recycled crustal component with high U/Pb had previously experienced only a short crustal residence time. The combined Pb and Sr data suggest that the depleted mantle beneath the Superior Province sampled by the complexes was initiated ∼3 Ga ago, in agreement with previously published results. Available data on carbonatites from the Fennoscandian Shield parallel the Canadian Shield isotope evolution pattern, although data from the southern hemisphere show considerable divergence, indicating regional variations. However, isotope evolution data from Precambrian marine precipitates suggest that major contributions of Sr to the oceans from sialic crust commenced ∼3 Ga ago. Assuming a complementary relationship between sialic crust and depleted mantle, the Canadian and European Sr data agree with average world-wide evolution of depleted mantle, and therefore appear to have more than regional significance. Models to reconcile the differences between the Nd and Sr results are considered. With present information several models are viable, some of which involve the first appearance of major volumes of depleted mantle around 3.0 Ga due to formation of sialic continental crust. The contrast between the Sr and Nd data may be due to differences in the way in which the bulk silicate Earth isotope evolution models for Nd compared to those for Sr and Pb are derived. Until the cause of the differences between Nd and the Sr and Pb data are better understood the presence of substantial volumes of sialic continental crust prior to ∼3.0 Ga ago is open to question.

Age, Cooling History, and Origin of Post-Collisional Leucogranites in the Karakoram Batholith; A Multi-System Isotope Study

U-Pb dating on zircon and monazite from different varieties of leucogranites in the Karakoram Batholith reveals two distinct pulses of plutonism at 25.5 +0.3/-0.8 and 21.4 +0.3/-0.6 Ma. In both granites, these minerals contain inherited components, documenting the presence of Precambrian material in the magma source. A crustal origin of the granites, and long crustal residence times of the source rocks are confirmed by highly negative $$\epsilon _{i}^{Nd}$$, strongly positive $$\epsilon _{i}^{Sr}$$, and $$T_{DM}^{ND}$$ in excess to 1 Ga. A biotite gneiss and a granitic dike from the Karakoram Metamorphic Complex south of the batholith yield very similar $$\epsilon _{i}^{Nd},\epsilon _{i}^{Sr}, T_{DM}^{ND}$$. In a Pb evolution diagram, initial isotopic compositions for both the granites and the rocks from the metamorphic complex plot far above the reference field for MORB documenting very close affinity to the Eurasian (Tibetan) continental crust. The striking similarity of isotope signatures between leuco...

Aluminous reaction textures in orthoamphibole‐bearing rocks: the pressure–temperature evolution of the high‐grade Proterozoic of the Bamble sector, south Norway

Aluminous reaction textures in orthoamphibole‐bearing rocks from the Froland area, Bamble, south Norway, record the prograde pressure–temperature path of the high‐grade Kongsbergian Orogeny (c. 1600–1500 Ma) and the low–mid amphibolite facies overprint during the Sveconorwegian Orogeny (c. 1100–1000 Ma). The rocks contain anthophyllite/gedrite, garnet, cordierite, biotite, quartz, andalusite, kyanite, Cr‐rich staurolite, tourmaline, ilmenite, rutile and corundum in a variety of parageneses. The P–T path is deduced from petrographic observations, mineral chemistry and zoning, geothermometry and (N)FMASH equilibria. The results indicate the sequence of metamorphic stages outlined below. (a) An M1 phase characterized by the presence of strongly deformed andalusite, gedrite and tourmaline.(b) An M2 phase with the development of kyanite after andalusite and the growth of staurolite associated with strong Na–Al–Mg zoning in orthoamphibole, indicating an increase in pressure (4 8 kbar) and temperature (500° 650°C).(c) Pressure decrease at high P (6–7 kbar) and high T (600–700 °C) during M3a with the production of cordierite ° Corundum between kyanite, staurolite and orthoamphibole and cordierite growth between corundum and orthoamphibole.(d) Temperature increase to 740 ± 60 °C and 7 kbar; static growth of garnet (M3b) at the metamorphic climax (peak T). The heat supply necessary to explain the temperature increase between the M3a and M3b phases is correlated with synkinematic enderbitic–charnockitic and basic intrusions in the Arendal granulite facies terrain.(e) M3b metamorphic conditions were followed by an initial isobaric cooling path (early M4) and late‐stage pressure decrease (late M4). Early M4 conditions of 6–7 kbar and 550–600 °C, assuming PH2O &lt; Ptotal are indicated by a retrograde talc–kyanite–quartz assemblage in late quartz–cordierite veins. Late M4 conditions of 3–4 kbar and 420–530 °C are inferred from a kyanite–andalusite–chlorite–quartz assemblage in vein‐cordierite. The M1–M3 stages are interpreted as being the result of the same metamorphic P–T path, which was caused by both tectonic and magmatic thickening. A prolonged crustal residence time is proposed for the Bamble sector before uplift during the later stages of M4 occurred.

Geochemistry and origin of Archean granites from the Black Hills, South Dakota

The Little Elk Granite (2549 Ma) and granite at Bear Mountain (BMG) (~2.5 Ga) of the Black Hills formed as a result of a collisional event along the eastern margin of the Wyoming Province during the late Archean. Geochemical modelling and Nd isotopic data indicate that the Little Elk Granite was generated by the partial melting of a slightly enriched (εNd = −1.07 to −3.69) granodioritic source that had a crustal residence time of at least 190 Ma. The medium-grained to pegmatitic, peraluminous, leucocratic BMG was produced by melting a long-lived (&gt;600 Ma), compositionally variable, enriched (εNd = −7.6 to −12.3) crustal source. This produced a volatile-rich, rare-earth-element-poor magma that experienced crystal–melt–volatile fractionation, which resulted in a lithologically complex granite.The production of volatile-rich granites, such as the BMG and the younger Harney Peak Granite (1715 Ma), is a function of the depositional and post-depositional tectonic environment of the sedimentary source rock. These environments control protolith composition and the occurrence of dehydration and melting reactions that are necessary for the generation of these volatile-rich leucocratic granites. These types of granites are commonly related to former continental–continental accretionary boundaries, and therefore their occurrence may be used as signatures of ancient continental suture zones.

C-O-H ratios of silicate melt inclusions in basalts from the Galapagos Spreading Center near 95°W: A laser decrepitation mass spectrometry study

Volatile ratios (primarily of H 2O and CO 2) in individual silicate melt (glass) inclusions in minerals have been analyzed using laser volatilization and mass spectrometry. A Nd-glass laser was used to produce 50-micrometer diameter pits in silicate melt inclusions. Released volatiles were analyzed directly with a computer-controlled quadrupole mass spectrometer. The detection limits for CO 2 and H 2O were on the order of 3 × 10 −14 and 3 × 10 −13 moles, respectively. The reproducibility for CO 2 H 2O was better than ±9%. The total range of volatile ratios from vitreous silicate glass inclusions contained in a suite of Galapagos lavas were: 0.018 to 1.193 for CO 2 H 2O ; 0.002 to 0.758 for CO H 2O ; 0 to 0.454 for CH 4 H 2O ; and 0 to 0.432 for Ar H 2O . The mean CO 2 H 2O from the propagating rift (0.245 ± 0.068) silicate glass inclusions is significantly lower than that of the actively failing rift (0.641 ± 0.241); this difference probably reflects different degrees of degassing during contrasting magmatic histories for the two regions. Relatively undifferentiated failing rift magmas must have relatively short crustal residence times prior to eruption and, therefore, have not undergone significant degassing of CO 2, as would appear to be the case for the more highly fractionated propagating rift magmas. The laser-mass spectrometric system described herein has the ability to act as a point-source probing device that can differentiate between the various volatile sites in minerals and rocks (as well as synthetic materials) on a micrometer scale.

Early Proterozoic crust-mantle interaction at a continental margin in northern Sweden

Abstract On the basis of recent geochemical and Sm-Nd isotopic data, alternative models for the early Proterozoic evolution of the Archaean craton of the Baltic Shield and its marginal areas in northern Sweden are discussed. Before the deposition of a craton cover and the subsequent Svecofennian orogenic activity, the Archaean areas probably formed a continuous domain. The 1.89-1.86 Ga old early Svecofennian granitoids found close to exposed Archaean rocks were created largely by the remobilization of Archaean crust. To the south of the Archaean Domain, however, chronologically equivalent granitoids are made up of matter with short crustal residence times. That area, the Skellefte district, represents a new addition to the continental crust of the Baltic Shield ∼1.90 Ga ago. The particulars of the crustal accretion process are still controversial. However, recent data indicate different tectonic developments in the Skellefte district and in the vicinity of the Archaean-Proterozoic boundary farther north. Apparently, the one-time edge of the Archaean craton was situated somewhere between these two areas, but appears to have had a rather more northerly position than previously assumed.