Stephen Moorbath

Crustal contributions to arc magmatism in the Andes of Central Chile

Fifteen andesite-dacite stratovolcanoes on the volcanic front of a single segment of the Andean arc show along-arc changes in isotopic and elemental ratios that demonstrate large crustal contributions to magma genesis. All 15 centers lie 90 km above the Benioff zone and 280±20 km from the trench axis. Rate and geometry of subduction and composition and age of subducted sediments and seafloor are nearly constant along the segment. Nonetheless, from S to N along the volcanic front (at 57.5% SiO2) K2O rises from 1.1 to 2.4 wt %, Ba from 300 to 600 ppm, and Ce from 25 to 50 ppm, whereas FeO*/MgO declines from >2.5 to 1.4. Ce/Yb and Hf/Lu triple northward, in part reflecting suppression of HREE enrichment by deep-crustal garnet. Rb, Cs, Th, and U contents all rise markedly from S to N, but Rb/Cs values double northward — opposite to prediction were the regional alkali enrichment controlled by sediment subduction. K/Rb drops steeply and scatters greatly within many (biotite-free) andesitic suites. Wide diversity in Zr/Hf, Zr/Rb, Ba/Ta, and Ba/La within and among neighboring suites (which lack zircon and alkali feldspar) largely reflects local variability of intracrustal (not slab or mantle) contributions. Pb-isotope data define a limited range that straddles the Stacey-Kramers line, is bracketed by values of local basement rocks, in part plots above the field of Nazca plate sediment, and shows no indication of a steep (mantle+sedimentary) Pb mixing trend. 87Sr/86Sr values rise northward from 0.7036 to 0.7057, and 143Nd/144Nd values drop from 0.5129 to 0.5125. A northward climb in basal elevation of volcanic-front edifices from 1350 m to 4500 m elevation coincides with a Bougueranomaly gradient from −95 to −295 mgal, interpreted to indicate thickening of the crust from 30–35 km to 50–60 km. Complementary to the thickening crust, the mantle wedge beneath the front thins northward from about 60 km to 30–40 km (as slab depth is constant). The thick northern crust contains an abundance of Paleozoic and Triassic rocks, whereas the proportion of younger arc-intrusive basement increases southward. Primitive basalts are unknown anywhere along the arc. Base-level isotopic and chemical values for each volcano are established by blending of subcrustal and deep-crustal magmas in zones of melting, assimilation, storage and homogenization (MASH) at the mantle-crust transition. Scavenging of mid-to upper-crustal silicic-alkalic melts and intracrustal AFC (prominent at the largest center) can subsequently modify ascending magmas, but the base-level geochemical signature at each center reflects the depth of its MASH zone and the age, composition, and proportional contribution of the lowermost crust.

Age significance of U–Th–Pb zircon data from early Archaean rocks of west Greenland — a reassessment based on combined ion-microprobe and imaging studies — reply

Abstract The geochronological evolution of early Archaean Amitsoq gneisses from southern West Greenland is reassessed here using the well-established cathodoluminescence imaging method both to reveal previously undocumented complex zircon growth histories and to control positioning of ion-microprobe U–Th–Pb analyses. Several of the gneisses typically contain zircon grains with ≥3.8 Ga cores of igneous origin, mostly completely mantled by growth banded magmatic zircon at ca. 3.65 Ga and in turn partly mantled by late Archaean metamorphic overgrowths at ca. 2.7 Ga. The ≥3.8 Ga cores are regarded as inherited, with apparent dates ranging down to ca. 3.65 Ga reflecting differential Pb loss. This contrasts with published views interpreting ≥3.8 Ga dates as emplacement ages of the host rock to the zircons, with all younger ages representing later Pb loss and/or metamorphic recrystallisation. Two of the analysed gneisses have discordant relationships to metasedimentary enclaves containing accessory apatite with graphite inclusions whose C-isotope ratios have been considered as biogenic in origin. The present data show that the minimum age of the enclaves is 3.65 Ga and not 3.85 Ga as previously claimed. This has important implications for the current debate on the possibility of temporal overlap of earliest life with a bolide impact scenario terminating at ≥3.8 Ga (as on the moon). We also discuss the implication of the discovery of highly complex internal structures in Amitsoq gneiss zircons for the interpretation of published, zircon-derived Lu–Hf information.

The significance of lead isotope studies in ancient, high-grade metamorphic basement complexes, as exemplified by the Lewisian rocks of Northwest Scotland

Abstract Lead isotope ratios, together with uranium and lead analyses, are reported for thirty-seven whole rock samples from the Precambrian Lewisian basement complex of Northwest Scotland. A fairly representative range of metamorphic and polymetamorphic gneisses, mainly in the amphibolite and pyroxene-granulv in this reconnaissance survey. The analysed rocks have a wide range of conventional radiometric dates in the range 2600 m.y. to 1600 m.y., depending on whether they are from the Scourian, Inverian or Laxfordian sectors of the Lewisian complex. The leads from these rocks cannot be interpreted by a single stage evolutionary model. The leads fall below the primary growth curve and scatter rather closely about a straight line on a plot of 206Pb/204Pb versus 207Pb/204Pb, which cuts the primary growth curve (with 238U/204Pb = μ = 8.68) at t1 = 2900 ± 100 m.y. and t2 = 0 m.y.. h This shows that all the analysed rocks underwent varying degrees of uranium depletion at 2900 ± 100 m.y. ago, presumably during the early pyroxene-granulite metamorphism. It also suggests that much of the Lewisian basement complex was already in existence about 2900 m.y. ago. Later metamorphisms of somewhat lower grade, such as the so-called Inverian and Laxfordian, have probably caused minor redistribution of uranium and/or lead leading to some scatter about the 206Pb/204Pb versus 207Pb/204Pb lead line, but, in general the uranium/lead ratios of these rocks have always been much lower than that indicated by the primary growth curve. It is suggested that lead isotope studies on metamorphic and polymetamorphic basement rocks can help to decide whether a given crustal segment represents essentially juvenile addition of material from the upper mantle, or reworked older crust.

Early Proterozoic crustal evolution in the birimian of Ghana: constraints from geochronology and isotope geochemistry

Abstract Representative suites of Ghanaian granitoids, metavolcanics and metasediments were selected from Birimian rock units for RbSr, Pb/Pb and SmNd analyses. Isotopic data on the igneous rocks provide evidence for a major Early Proterozoic crust-forming episode which took place over a maximum time interval from ∼ 2.3 to ∼ 2.0 Ga by a differention from a depleted mantle source. There is little evidence for the involvement of significantly older (e.g. Archaean) crust in the genesis of the igneous rock units. The only exception so far found is the Winneba granitoid, which appears to contain a magmatic contribution from presumably underlying Archaean basement. SmNd model ages indicates that Birimian sediments were derived from adjacent penecontemporaneous volcanic belts. The Birimian of Ghana forms part of a major Proterozoic (Eburnean) episode of juvenile crustal accretion which has been recognised in the surrounding areas of Mauretania, Senegal, Ivory Coast, Burkina Faso, Mali and Niger, where it has been dated at 2.1–2.2 ga (abouchami et al.) The Birimian terranes bridge a major gap in mantle activity and associated crustal evolution for a period considered as quiescent on other continents.

Petrography, chemistry and isotopic ages of Peninsular Gneiss, Dharwar acid volcanic rocks and the Chitradurga granite with special reference to the late Archean evolution of the Karnataka craton, southern India

Abstract New petrographic, major and trace element, and isotopic age data are presented for four suites of rocks from the low- to medium-grade part of the South Indian Archaean craton in central Karnataka. Two suites are from the Peninsular Gneiss basement to the late Archaean Dharwar Supergroup, the third is a suite of Dharwar potassic rhyolites and the fourth is from the Chitradurga Granite which has intrusive contacts with the Dharwar Supergroup. The Chikmagalur Granite (granodiorite), which with its host gneisses forms part of the basement (Peninsular Gneiss) to the Dharwar Supergroup in the Bababudan basin, yields RbSr and Pb/Pb whole-rock isochron dates of 3080 ± 110 Ma and 3175 ± 45 Ma, respectively, an initial 87 Sr 86 Sr ratio of 0.7013 ± 0.0009 and a μ1 value ( 238 U 204 Pb ) of 7.98. Host tonalitic gneisses to the Chikmagalur Granite give an RbSr whole-rock isochron date of 3060 ± 160 Ma with an initial 87 Sr 86 Sr ratio of 0.7015 ± 0.0004 and a Pb/Pb whole-rock isochron date of 3190 ± 100 Ma with a model μ1 value of 8.00. A suite of granites and granodioritic-tonalitic gneisses (Chitradurga granitic rocks and gneisses) forming part of the Peninsular Gneiss basement to the Dharwar Supergroup west of the Chitradurga belt yields RbSr and Pb/Pb whole-rock isochron dates of 2970 ± 100 Ma and 3028 ± 28 Ma, respectively, with an initial 87 Sr 86 Sr ratio of 0.7035 ± 0.0013 and a model μ1 value of 7.62. The suite of Dharwar acid volcanic rocks from north of Honnali defines a poorly fitted 207 Pb 206 Pb linear array (MSWD 15.8) which gives an apparent age of 2565 ± 28 Ma and a model μ1 value of 7.46. The poor fit of this ‘isochron’ shows that a closed system was not maintained, a conclusion supported by high K abundances which are attributed to a syn- or post-magmatic ion exchange process. The Chitradurga Granite which intrudes the older rocks of the Dharwar Supergroup in the Chitradurga belt yields a well-fitted Pb/Pb whole-rock isochron date of 2605 ± 18 Ma with a model μ1 value of 7.68. The UPb relations in these suites from Karnataka are in marked contrast with the commonly severe U depletion and, consequently, unradiogenic Pb isotopic compositions observed in deeply eroded, high-grade Archaean gneiss terranes such as those bordering the North Atlantic. This contrast indicates that the cratonic rocks in central Karnataka represent a relatively high level in the original Archaean continental crust.

Ages, isotopes and evolution of Precambrian continental crust☆

Abstract It is certain that typical continental crust, of unknown extent and thickness, but comprising a wide variety of igneous, sedimentary and metamorphic rocks, mostly characteristic of the “granite—greenstone” association, was in existence by about 3700–3800 m.y. ago. Rocks in the general age range 2600–2800 m.y. are very widely distributed. This was undoubtedly one of the major rock-forming episodes in earth history. In this paper, geochronological, isotopic and much other evidence is cited in support of the hypothesis that irreversible chemical differentiation of part of the upper mantle has produced new continental, sialic crust over the whole of geological time. It is possible that the addition of new sial occurred during relatively short (ca. 100–200 m.y.), possibly worldwide mantle differentiation episodes, widely separated in time, during which the new sial also underwent almost synchronous, rapid igneous, metamorphic and geochemical differentiation. These events are termed “accretion-differentiation” episodes. Continental growth greatly dominates over continental recycling, because of the permanence of sialic crust. A basically uniformitarian approach to earth history is favoured for the past 3800 m.y.

Isotopic dating of very early Precambrian amphibolite facies gneisses from the Godthaab district, West Greenland

Abstract Amphibolite-facies feldspathic gneisses (the Amitsoq gneisses) from the Godthaab area of West Greenland give hitherto unique age and isotope data, including (i) a Rb-Sr whole-rock isochron age of 3980 ± 170 my. (t 1/2 = 5.0 × 10 10 y) with an initial 87 Sr/ 86 Sr ratio of 0.6992 ± 0.0010, (ii) a Pb-Pb whole-rock isochron age of 3620 ± 100 my, and (iii) extremely unradiogenic whole-rock leads, with 206 Pb/ 204 Pb and 207 Pb/ 204 Pb values extending down to 11.51 and 13.14 respectively. Taken together, these data provide the first direct evidence for the existence of a granitic crust so early in the Earths history. The isotopic evidence also demonstrates that these rocks could not have had a lengthy (i.e. more than a few tens of millions of years) history prior to the Rb-Sr isochron age. Possible interpretations of the isochron ages are discussed in detail. Several other petrologically distinctive rocks from the same area are shown to have been affected by thermal events somewhere in the range 2000–2800 my ago.

KAr ages of the oldest exposed rocks in Iceland

Abstract K Ar ages are reported for the stratigraphically oldest exposed lavas in Iceland. The mean values are 12.5 ± 0.2 m.y. for eastern Iceland and 16.0 ± 0.3 m.y. for northwestern Iceland, or Middle Miocene. The significance of these ages is discussed in relation to the stratigraphy, to the hypothesis of ocean floor spreading and crustal drift, and to the overall rate of lava production. Several additional age determinations from Iceland are presented which supplement and confirm previously published age data.

Dating the oldest terrestrial rocks — fact and fiction

Abstract This paper is a review of well-documented geochronological and related isotopic evidence for the age and total crustal residence time of the oldest known terrestrial rocks, mainly using Rb/1bSr, Sm/1bNd and U/1bPb decay schemes. The oldest known stable sialic crust of true continental character, predominantly composed of calc-alkaline orthogneisses, was produced during major mantle-differentiation some 3.7−3.5 Ga ago and is represented on several continents. Enclaves of older supracrustal rocks, as well as a major remnant of a volcano-sedimentary sequence of greenstone belt affinity dated at ∼3.8 Ga, are preserved within the surrounding gneisses in southern West Greenland. Recent ionmicroprobe evidence for ∼4.1-Ga-old detrital zircon grains in a younger early Archaean metasedimentary sequence in Western Australia is also discussed. Several cases are reviewed where claims for early Archaean (> 3.4 Ga) dates are believed to be erroneous and to result from oversimplistic and incorrect interpretation of isotopic data. Some of the geological and geochemical mechanisms which have given rise to claims for spuriously old Sm/1bNd, U/1bPb and Pb/Pb dates are critically discussed. To supplement published data, we present several new sets of whole-rock Sm/1bNd and Pb/Pb isotopic data from West Greenland, Labrador, Zimbabwe and eastern India relevant to the overall topic of this review.

The Nevados de Payachata volcanic region (18°S/69°W, N. Chile) II. Evidence for widespread crustal involvement in Andean magmatism

Volcanism extending over 11 Ma is represented in the rocks of the Nevados de Payachata region, culminating in the formation of two large composite stratocones within the last 500 000 years. Chemically distinct mafic magmas are erupted at a number of parasitic centers. These cannot be related to each other by crystal fractionation and do not appear to be direct parents for the differentiated suites of the composite cones. Two distinct trends are defined by the intermediate and evolved rocks; a high LILE (large ion lithophile element), TiO2 and Ce/Yb lineage among the youngest rocks (including the two major stratocones), and a more typical calc-alkaline trend among the older (>1 Ma) rock types. Within individual volcanic centers, differentiation involves fractionation of plagioclase, pyroxene and hornblende, with biotite and K-feldspar in the more-evolved rock types. Isotopic compositions (Sr, Pb, Nd, O) vary little with differentiation from basaltic andesite to rhyolite, or with age. Contamination during differentiation from basalt to rhyolite may occur, but the most mafic rocks erupted in the region are already enriched in incompatible trace elements and therefore may be insensitive to the effects of interaction with the crust. The majority of data are similar to “baseline” compositions (Cenozoic parental magmas) from other parts of the central Andes and may reflect a relatively homogeneous magma source (or source mixture) throughout this central volcanic zone (CVZ), which is distinct from the southern and northern Andes, and from island-arc volcanic rocks.The detailed study of Nevados de Payachata serves as a useful reference against which to assess magmatism in general in the CVZ. The possibility that central Andean magmas are generated from an enriched subcontinental-lithosphere mantle wedge is rejected on the basis of: (1) thermal considerations (subcontinental mantle lithosphere is probably cold and refractory); (2) lack of consistency between the tectonic history of the region and geochemical variations through time. Instead, parental magmas in the CVZ are thought to be generated by mixing between normal arc magmas originating in the depleted mantle wedge followed by contamination and homogenization with lower crustal melts. In the central Andes, the extent of contamination increased greatly as the crust thickened due to crustal shortening within the last 20 Ma, the thicker crust providing an effective filter to trap and differentiate magma batches repeatedly during ascent.