The sources of igneous rocks in the continental crust are elusive, but they may be traced by radiogenic isotopes, which convey a message about the age and composition of the concealed parts of the continent. Mar 22, 2016. Lee Wows You With A Historical Music Moment! - America's Got Talent 2019 - Duration: 8:17. America's Got Talent Recommended for you.
The sources of igneous rocks in the continental crust are elusive, but they may be traced by radiogenic isotopes, which convey a message about the age and composition of the concealed parts of the continent. We investigated the Hf-isotope composition of zircon in ten rocks from central and southern Sweden. Two felsic metavolcanic rocks and two metagabbros (ca.
1.89 Ga) from Bergslagen, southern Sweden, show ε Hf(t) ranges of −1.8 to +5.1 and +2.6 to +6.8, respectively, suggesting that juvenile sources have contributed to both. A 1.85 Ga granite from southern Bergslagen shows a ε Hf(t) range of −2.6 to +4.6 for magmatic zircons, but both highly negative and positive values for inherited grains, providing evidence for both Archean and juvenile crustal sources. These and previous data confirm the existence of juvenile proto-Svecofennian crust (. The Hf-isotope ratios of individual zircon crystals record heterogeneities in the magmas of various igneous rock types at a much higher spatial resolution than do whole-rock isotopic data (e.g.,;;; ). Although whole-rock data yield information on the characteristics of the magma source(s) at the scale of whole-rock samples within a given volume of a rock suite, this approach averages out any evidence of mixed provenance within a single specimen.
Source components of contrasting isotopic composition may be preserved in a magma on a very local scale (e.g., the size of a hand specimen) within early-crystallized minerals that are stable enough to have survived subsequent stirring and mixing (e.g.,;;; ). Initial Hf-isotope data in zircons separated from one sample may thus provide a wider spectrum of values than a suite of whole-rock samples (although with a somewhat lower analytical precision), even approaching the end-member source compositions (e.g.,;,;; ). In addition, U-Pb and Lu-Hf spot analysis by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) on cores and overgrowth zones in zircons from igneous rocks yields combined information on the spread in protolith and assimilant ages and the isotopic composition of the magmas from which they crystallized (e.g.,;; ). This provides a powerful tool for unraveling the history of crustal tracts. There exists a limited volume of whole-rock and multigrain-zircon Hf-isotope data from the Fennoscandian Shield, including various granitoid (; ) and mafic suites (;, ); these data essentially replicate the information provided by the more abundant Nd-isotope data (see compilations in, e.g.,;,;;; ). Recent in situ zircon-Hf data come mainly from the SW part of the shield, where 1.7–0.9 Ga granitoids yield initial ε Hf compositions ranging from depleted mantle to somewhat below chondritic uniform reservoir (CHUR) (;,; ), and even to highly negative values for some late Sveconorwegian grains (, ). Only a few studies have reported such data from rocks of the Archean to Paleoproterozoic part of the shield (;;;; ).
Here, we present new in situ zircon Hf-isotope data for 1.90–1.85 Ga mafic and felsic rocks from the Bergslagen region, southern Sweden; these data add further constraints on the origin of the continental crust in this part of the Fennoscandian Shield. In addition, data are reported for five 1.53–1.50 Ga rapakivi intrusions in central Sweden, yielding information pertaining to the sources for this magmatism. The results are combined with previous isotopic data to build an integrated crustal model for the western and southern parts of the Svecofennian Domain. GEOLOGICAL SETTING AND SAMPLES. The classical Bergslagen mining region in eastern southern Sweden consists of 1.91–1.87 Ga ore-bearing intercalated metavolcanic and metasedimentary rocks. The (mainly felsic) volcanic rocks dominate in the west and north, while the metasedimentary rocks are dominant in the SE part of the region (e.g.,; ). The supracrustal rocks were intruded by an early Svecofennian granite-granodiorite-tonalite-gabbro suite at 1.89–1.86 Ga, and a suite of late Svecofennian, migmatite-related granites and pegmatites at 1.85–1.75 Ga (summarized in, e.g.,;; ).
In the south and west, the Svecofennian successions are truncated by 1.85–1.65 Ga granitic-quartz monzonitic-monzodioritic-gabbroic intrusions of the Transscandinavian Igneous Belt (e.g., ). The Transscandinavian Igneous Belt is commonly subdivided into intrusive episodes, of which the earliest (ca. 1.85–1.83 Ga) occurred mainly along the SW Svecofennian border zone (e.g.,;; ). The second, and dominant, episode (1.81–1.75 Ga) followed mainly farther away from the Svecofennian margin (e.g.,; ), while the third episode (1.71–1.65 Ga) was emplaced even farther to the west and north (e.g.,;;; ). Five samples from Bergslagen were studied: two mafic intrusions, two felsic volcanic rocks, and one sample of the earliest Transscandinavian Igneous Belt generation. The petrography and secondary ion mass spectrometry (SIMS) geochronology of these samples are described in.
All samples have been metamorphosed in middle to upper amphibolite facies. The two mafic intrusive rocks (R89-14: 1887 ± 5 and UB98-29: 1895 ± 5 Ma) were sampled from western Bergslagen, the two felsic volcanic rocks (U97-1: 1888 ± 12 and U97-3: 1892 ± 7 Ma) were sampled from the eastern part, and the early Transscandinavian Igneous Belt augen gneiss (Fin2: 1855 ± 6 Ma) came from the southern part. The five zircon samples from the Bergslagen area were previously analyzed for U-Th-Pb using the SIMS technique. Our Lu-Hf analyses were obtained in the same analytical spots as were used for the previous U-Pb analyses.
Thus, the age of zircon growth was known for each of these spots. Crystallization ages for the five rapakivi intrusions were previously determined only by zircon U-Pb TIMS geochronology (; ). The great majority of crystals from these intrusions show uncomplicated magmatic growth zoning, and such crystals were selected both for the previous TIMS analysis and for the present Lu-Hf study. Hf-isotope analyses were performed at National Key Centre for Geochemical Evolution and Metallogeny of Continents of the Australian Research Council (GEMOC ARC), Macquarie University, Australia. The 176Hf/ 177Hf ratios in zircon were measured with a New Wave Research 213 nm laser-ablation microprobe attached to a Nu Plasma multicollector ICP-MS.
The analytical methods, including extensive data on the analysis of standard solutions and zircons, were discussed in detail by, ). Data were processed using the Nu Plasma time-resolved analysis software, which allows selection of the most stable part of the ablation signal. Analyses were carried out at 5 Hz frequency with a beam diameter 55 μm and energies around 0.1 mJ per pulse. Background was measured for 60 s prior to ablation. The length of the analysis varied between 30 and 140 s, depending on the thickness of the grains. Repeated analyses of the Mud Tank zircon (long-term running average 176Hf/ 177Hf = 0.282523 ± 43; ) and the 91500 zircon (long-term running average 176Hf/ 177Hf = 0.282307 ± 58; ) were used to monitor data quality, translating to about ±2 ε units. For the decay constant of 176Lu, a value of 1.867 × 10 −11 yr −1 (, ) was applied.
The present-day chondritic values used were 176Lu/ 177Hf = 0.0336 and 176Hf/ 177Hf = 0.282785. For the depleted mantle reference curve, the straight-line model of was adopted, updated by the more recent values for CHUR and the decay constant of 176Lu, which yields a present-day value of 176Hf/ 177Hf = 0.28325 (ε Hf = +16.4), similar to mid-ocean-ridge basalt (MORB), and a 176Lu/ 177Hf DM value of 0.0388. The results of all analyses are given in and plotted in ε Hf versus time diagrams in. Even though the 207Pb/ 206Pb age was known for each individual spot for the Bergslagen zircons, the initial ε Hf value for each analysis from the magmatic parts of crystals is plotted at the calculated crystallization age of the rock. This is because of the relatively large error in the individual SIMS analyses, variable degrees of discordance, and possible nonzero lower intercepts.
However, inherited cores and overgrowths are plotted at their individual 207Pb/ 206Pb ages. The initial ε Hf values of the rapakivi zircons are plotted at the calculated TIMS age of crystallization. The homogeneous zoning of the zircons and the coherency of the Hf isotopic results support a magmatic origin for the analyzed crystals. In reference to the data and discussion, one should bear in mind the analytical precision of ≤2 ε units, which, however, will not alter the interpretations and conclusions. Following rifting of the Archean craton margin in the NE, new crust started to form in several arc systems in successive stages outboard of the craton from ca. 2.1 Ga (;; ), creating early “microcontinents” in the period 2.1–1.93 Ga (, ); little of this older crust is exposed at the present erosion level. This proto-Svecofennian crust was strongly reworked from 1.91 to 1.86 Ga, when most of the presently exposed rocks in the domain were formed.
The preexistence of the proto-Svecofennian crust is substantiated by a few outcropping 1.95–1.91 Ga igneous rocks, and especially by the dominance of juvenile (. Except for one grain, the present data for zircons from the felsic metavolcanic rocks, and most of the igneous zircons of the Transscandinavian Igneous Belt granite, show positive initial ε Hf values. Sample U97-3 shows slightly lower ε Hf values, compared with U97-1, including one slightly below CHUR. This may be correlated with its inferred character as a redeposited volcanogenic sediment ; Svecofennian metasediments typically contain higher proportions of older material compared with the meta-igneous rocks (see summary in ). The total range of values is ∼7 ε units of the ∼16 ε units between DM and the upper limit of the evolution for the Fennoscandian Archean crust, and they all fall entirely on the depleted (high-ε) side of CHUR. The present data corroborate previous juvenile bulk-rock Hf-isotope data from Svecofennian granites in Bergslagen and southern Finland (; ) and zircon Hf-isotope data from Svecofennian and Transscandinavian Igneous Belt granites in Sweden (; ). This restricted range of positive initial ratios does not lend support to a model of mixing between Archean and DM sources, which would tend to generate a larger spread in initial ratios, but rather suggests a dominance of sources separated from the mantle in early Paleoproterozoic time (cf.
Similarly, Hf data for zircons from Svecofennian granitoids in southern Finland tend to be relatively juvenile in the west but carry higher proportions of Archean signatures toward the craton margin in the east (;; ). Inherited cores of zircons from the Transscandinavian Igneous Belt augen gneiss span large ranges in both age and initial ε Hf. However, most of those in the age range 2.1 to 1.9 Ga have initial ε Hf values on the juvenile side (+0.6 to +3.6).
Only one originally crystallized at ca. 1.97 Ga from a magma derived from Archean crust (ε Hf = −16.1). The five cores with 207Pb/ 206Pb ages in the range 2.39 to 2.95 Ga show initial ε Hf in the range −2.4 to −14.6, and these values are typical for Fennoscandian Archean crust. Even if the exact ages of these grains are uncertain, due to discordance, they most certainly contain dominantly Archean Hf. Similarly, the ages are uncertain for the two juvenile analyses at 2.24 and 2.35 Ga, and they may also in reality be Archean.
One of these has a very radiogenic Hf-isotope composition. Due to the low Lu/Hf ratio in zircon, this depleted character will be present in this sample irrespective of age, and it thus shows the existence of very juvenile material in the crustal mixture at the time of its formation. Suggested that the limited spread in initial ε Hf data for zircons from Svecofennian and Transscandinavian Igneous Belt granitoids could be explained by derivation from preexisting juvenile Svecofennian crust that is approximately defined by the evolution ε Hf(1.88) = 2 ± 3 and 176Lu/ 177Hf ≈ 0.015. The presence of a juvenile Svecofennian protocrust is supported by the positive ε Hf values of 2.2–1.9 Ga inherited zircons in granites (;; ). The coexistence of early Paleoproterozoic inherited grains with both DM and Archean initial Hf isotopic compositions suggests that the earliest Svecofennian protocrust formed from a mixture of these components after 2.2 Ga. This protocrust probably was created in early arc systems of variable maturity from a depleted to increasingly enriched mantle, mixed with subordinate amounts of components derived from the Archean craton (cf.;; ). The development of this protocrust involved the creation and accretion of several arc systems, as well as reworking of early arcs into mature arcs (microcontinents), and collisions with the pre-accreted continent over such extended time (see, e.g.,;, ).
The Hf-isotopic evolution of the Svecofennian protocrust can be broadly constrained to ε Hf(1.89) = +3 ± 3 and 176Lu/ 177Hf ≈ 0.018, based on the presently available Svecofennian data, and data from Transscandinavian Igneous Belt granitoids that are considered to be derived from the Svecofennian crust. This 176Lu/ 177Hf ratio is higher than that of estimates for the average continental crust (0.010–0.015), but it is in the range estimated for the lower crust (0.015–0.020) (e.g.,;; ). A relatively mafic composition, similar to that of the lower crust, would be anticipated for such a juvenile mantle–derived protocrust. This evolution trend is slightly revised from that of, taking into account the additional data. Sub-Svecofennian Mantle. The data for the two mafic samples are the first reported for zircons in early Svecofennian mafic rocks.
Even if the overlap with zircons from the felsic rocks is substantial, their ε Hf in general ranges to higher values. The range indicates mildly to relatively strongly depleted sources, though not as depleted as the DM. Initial ε Nd data for early Svecofennian mafic rocks range from DM to values slightly below CHUR, but most are “mildly depleted” (see compilations in; ). These data, and particularly the dominant initial ε Nd (+1 to +2) of mafic Transscandinavian Igneous Belt rocks in southern Sweden (e.g.,;;; ), suggest the widespread occurrence of “mildly depleted” mantle sources in this part of the shield. Similarly, in southern Finland, mafic 1.9–1.8 Ga rocks have chondritic to mildly positive ε Nd values (e.g.,;; ).
This, together with MORB-like, or lower, contents of high field strength elements and enrichments in light rare earth and large ion lithophile elements relative to MORB, led, to suggest that the sources for this magmatism represent previously depleted mantle that became enriched during Svecofennian (. Suggested a heterogeneous distribution of such mantle sections to explain the variation from slightly subchondritic to strongly depleted initial ε Nd values in the early Svecofennian mafic rocks. An evolution of the sub-Svecofennian mantle defined by 147Sm/ 144Nd = 0.155 ± 0.015 would encompass the initial ε Nd ranges for most post-Svecofennian mafic suites in the Svecofennian Domain, and it is compatible with the 147Sm/ 144Nd ratios measured in enriched mantle xenoliths from elsewhere (e.g.,;;; ). Although the initial ε Hf values of various 1.6–0.28 Ga mafic rock suites in southern Fennoscandia show variations in detail, compatible with variable degrees of source enrichment , they broadly follow a common evolution that was defined by as ε Hf(1.60) = 3 ± 3 and 176Lu/ 177Hf?≈ 0.033 (excluding the more depleted Central Svecofennian Dolerite Group), i.e., essentially parallel to CHUR. However, the present early Svecofennian data show slightly more depleted initial values and constrain, together with the younger intrusions from southern Fennoscandia, an evolution of approximately: ε Hf(1.89) = 4.5 ± 2.5 and 176Lu/ 177Hf?≈ 0.0315 , representing an early Svecofennian “mildly depleted mantle,” similar to the one suggested by Nd isotopes for the Transscandinavian Igneous Belt and many early Svecofennian rocks (e.g.,; ). Thus, the early Svecofennian mafic rocks were derived from depleted mantle sections that acquired their variably mildly depleted characters through additions of fluids and melts carrying an Archean isotopic imprint into the mantle wedge by subduction during arc and microcontinent assembly and reworking in proto-Svecofennian time (. With the accumulation of more data on early Svecofennian mafic rocks, initial ε Hf values are likely to show an increased spread, reflecting variously enriched sections of the sub-Svecofennian mantle.
This is tentatively demonstrated by Hf-isotope data from 2.15 to 1.97 Ga mafic intrusions in Finland , some of which reflect increased contributions of Archean crustal components toward the craton in the east. However, if the Hf-isotope data remain similar to the Nd-isotope data, most of the data are expected to be “mildly depleted,” roughly coinciding with the evolution sketched in.
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Source Variations in Fennoscandian Rapakivi Complexes. The zircon data for the central Swedish rapakivi granitoids show an overall range in initial ε Hf values of ∼9 ε units (−10 to −1.5), and a within-sample variation of less than 3.5 ε units, while the total spread between DM and the Fennoscandian Archean crust at 1.5 Ga is ∼22 units. Thus, the Hf data allow an interpretation that the zircons crystallized from thoroughly mixed magmas with variable proportions of crustal- and mantle-derived material, or from magmas with an origin entirely within a strongly enriched mantle. However, the dominantly granitic compositions (cf., ), relatively unradiogenic Hf isotopes, and small and characteristic within-sample Hf isotopic variations favor distinct crustal sources for the granitoids of each complex. Initial ε Hf values in zircon from early Svecofennian (ca. 1.89 Ga) felsic volcanic rocks range from −1.8 to +5.1, similar to the range in early Svecofennian granitoids, supporting a dominance of juvenile Proterozoic sources for such magmas in the southern Svecofennian province.
This implies a reworking of material separated from the mantle. This work was supported by ARC (Australian Research Council) Linkage project LP0776637 to S.Y. The analytical work at the ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC) used instrumentation purchased and supported with funding from ARC, DEST, Macquarie University, and industry. This is publication 782 from ARC National Key Centre GEMOC, and publication 12 of the ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS). The reviews of P.J. Patchett and one anonymous reviewer helped to improve the paper.
(A–C) ε Hf versus time diagram for the present samples and other relevant data. Chondritic uniform reservoir (CHUR) is after.
Depleted mantle (DM) is after, modified by the CHUR data of. Upper limit of Fennoscandian Archean crust is as defined. Green ellipse in A encompasses the zircon data for early Svecofennian mafic rocks of this study and includes the data of. The ellipse for early Svecofennian granites includes data from Bergslagen and south-central Finland (; ). The ellipse for late Svecofennian granites includes data from,. Violet diamonds—Transscandinavian Igneous Belt (TIB) granitoids.
Various red and green squares—Finnish rapakivi granites and their associated mafic rocks, respectively (; ). Crosses—various suites of younger mafic intrusions in the central part of the Fennoscandian Shield. The latter data are broadly encompassed by the evolutionary field for the Paleoproterozoic Fennoscandian “mildly depleted” subcontinental mantle (MDM), approximated by: ε Hf(1.90) = 4.5 ± 2.5 and 176Lu/ 177Hf = 0.0315.
SJPC—Svecofennian juvenile protocrust, representing new crust formed after 2.2 Ga in the Svecofennian Domain, partly by mixing with Archean material (indicated by large arrows). Post–2.0 Ga felsic crust in the southern Svecofennian province is largely formed from this juvenile crust and lies within an evolutionary field defined by: ε Hf(1.90) = 3 ± 3 and 176Lu/ 177Hf = 0.018, including Transscandinavian Igneous Belt granitoids and Finnish rapakivi granites. Frames in A outline areas enlarged in B and C.
Blue arrow in B indicates minor Archean contributions to Svecofennian granitoids plotting below the juvenile crust, supplied mainly through melting of metasediments. Arrows in C indicate Svecofennian and Archean contributions to the rapakivi granitoids from various local mixtures of meta-igneous sources. (A–C) ε Hf versus time diagram for the present samples and other relevant data. Chondritic uniform reservoir (CHUR) is after. Depleted mantle (DM) is after, modified by the CHUR data of. Upper limit of Fennoscandian Archean crust is as defined.
Green ellipse in A encompasses the zircon data for early Svecofennian mafic rocks of this study and includes the data of. The ellipse for early Svecofennian granites includes data from Bergslagen and south-central Finland (; ).
The ellipse for late Svecofennian granites includes data from,. Violet diamonds—Transscandinavian Igneous Belt (TIB) granitoids. Various red and green squares—Finnish rapakivi granites and their associated mafic rocks, respectively (; ). Crosses—various suites of younger mafic intrusions in the central part of the Fennoscandian Shield. The latter data are broadly encompassed by the evolutionary field for the Paleoproterozoic Fennoscandian “mildly depleted” subcontinental mantle (MDM), approximated by: ε Hf(1.90) = 4.5 ± 2.5 and 176Lu/ 177Hf = 0.0315. SJPC—Svecofennian juvenile protocrust, representing new crust formed after 2.2 Ga in the Svecofennian Domain, partly by mixing with Archean material (indicated by large arrows). Post–2.0 Ga felsic crust in the southern Svecofennian province is largely formed from this juvenile crust and lies within an evolutionary field defined by: ε Hf(1.90) = 3 ± 3 and 176Lu/ 177Hf = 0.018, including Transscandinavian Igneous Belt granitoids and Finnish rapakivi granites.
Frames in A outline areas enlarged in B and C. Blue arrow in B indicates minor Archean contributions to Svecofennian granitoids plotting below the juvenile crust, supplied mainly through melting of metasediments. Arrows in C indicate Svecofennian and Archean contributions to the rapakivi granitoids from various local mixtures of meta-igneous sources.