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Crustal evolution of the Arabian–Nubian Shield: Insights from zircon geochronology and Nd–Hf–O isotopes
Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Museum of Natural History.ORCID iD: 0000-0003-3654-3149
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Arabian–Nubian Shield (ANS) represents a major site of juvenile Neoproterozoic crustal addition on Earth and documents Neoproterozoic tectonics bracketed by two supercontinent cycles, namely the fragmentation of Rodinia and the amalgamation of Gondwana. There is general consensus that the ANS formed by juvenile magmatic arc accretion and subsequent shield–wide post–tectonic magmatism. However, detailed understanding about the timing of events and the nature of magma sources in parts of the shield are lacking. To date, there are no isotopic data from the Paleozoic sedimentary sequences of the ANS, except those from the northern part. New zircon U–Pb, δ18O and whole–rock Nd isotopes are presented for plutonic rocks from the eastern Ethiopia, Yemen and southernmost Arabian Shield in Saudi Arabia. This thesis also presents the first combined in situ zircon U–Pb–O–Hf isotope data on the Cambrian–Ordovician sandstones of the Arabian Shield. The results are used to elucidate the crustal evolution of these parts of the ANS and to evaluate terrane correlations. Specifically, the nature of crustal growth, i.e., relative proportions of juvenile magmatic additions vs. crustal reworking, nature of the magma source and mechanism of crust formation (plume material vs. subduction zone enrichment) and understanding the provenance of the Cambrian–Ordovician sandstone sequences were important research questions addressed.

The results from Paper I suggest that the eastern Ethiopian Precambrian basement is dominated by reworking of pre-Neoproterozoic supracrustal material unlike contemporaneous rocks in the remaining parts of Ethiopia— indicating the presence of two distinct lithospheric blocks of contrasting isotopic compositions in Ethiopia. Metamorphic age distributions suggest that the eastern Ethiopian block was amalgamated with the juvenile Western Ethiopian Shield during ca. 580–550 Ma. Importantly, the suture between them may represent the northern continuation of a major suture identified further south in Africa along which Gondwana amalgamated. Similarly, the Abas terrane in Yemen (Paper II) is dominated by reworking of pre–Neoproterozoic crust and shows age and isotopic compositions that are inconsistent with the Afif terrane of Saudi Arabia, precluding correlation between the two regions. The trace element systematics of plutonic rocks from the southernmost Arabian Shield (paper III) point to enrichment due to subduction component, bear no evidence of a plume component, and are consistent with the adakite-like chemistry of some of the subduction–related plutonic samples. This reinforces the notion that the shield grew through juvenile magmatic arc additions. The combined zircon U–Pb–O–Hf data of the Cambrian–Ordovician sandstones (Paper IV) indicate their derivation from both the adjacent juvenile ANS and the more southerly crustal blocks that are dominated by reworking of pre–Neoproterozoic crust. The remarkable similarity in age spectra and homogeneity of Cambrian sandstones deposited across the northern margin of Gondwana point to continental–scale sediment mixing and dispersal regulated by the supercontinent cycle.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University , 2017. , 27 p.
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper, 367
Keyword [en]
Gondwana, Arabian–Nubian Shield, Azania, zircon U–Pb–O–Hf isotopes, Nd isotopes, Cambrian Sandstones, juvenile, pre–Neoproterozoic, reworking
National Category
Earth and Related Environmental Sciences
Research subject
Geology
Identifiers
URN: urn:nbn:se:su:diva-145479ISBN: 978-91-7649-938-2 (print)ISBN: 978-91-7649-939-9 (electronic)OAI: oai:DiVA.org:su-145479DiVA: diva2:1129701
Public defence
2017-09-20, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2017-08-28 Created: 2017-08-06 Last updated: 2017-08-28Bibliographically approved
List of papers
1. Zircon U-Pb ages, delta O-18 and whole-rock Nd isotopic compositions of the Dire Dawa Precambrian basement, eastern Ethiopia: implications for the assembly of Gondwana
Open this publication in new window or tab >>Zircon U-Pb ages, delta O-18 and whole-rock Nd isotopic compositions of the Dire Dawa Precambrian basement, eastern Ethiopia: implications for the assembly of Gondwana
2017 (English)In: Journal of the Geological Society, ISSN 0016-7649, E-ISSN 2041-479X, Vol. 174, no 1, 142-156 p.Article in journal (Refereed) Published
Abstract [en]

New high spatial resolution secondary ion mass spectrometry (SIMS) zircon dating from the Dire Dawa Precambrian basement yields crystallization ages at c. 790 Ma and 600 - 560 Ma. Two of the youngest samples are pervasively deformed, indicating that orogenesis continued until c. 560 Ma. SIMS delta O-18(zrn) shows bimodality, with the oldest sample (c. 790 Ma) and inherited zircons of that age in the younger samples having values of 7.8 - 9.6%, whereas the Ediacaran samples have delta O-18(zrn) values of 4.9 - 7.2%. These delta O-18(zrn) ratios are higher than mantle values and indicate a supracrustal input to the source of the Dire Dawa granitoids. All samples have unradiogenic epsilon(Nd)(t) values of -10.3 to -5.8 and Nd model ages of 1.72-1.42 Ga. These attributes suggest that the Dire Dawa granitoids were mostly derived from reworking of long-lived crustal sources. The occurrence of c. 580 - 550 Ma orogenesis in both the Dire Dawa basement and the juvenile Western Ethiopian Shield and the confinement of c. 630 Ma metamorphism to only the latter indicate that these two lithospheric blocks of contrasting isotopic compositions amalgamated at c. 580 - 550 Ma. This suggests that the Mozambique Ocean, which separated these two lithospheric blocks, was completely consumed during the late Ediacaran to early Cambrian.

National Category
Earth and Related Environmental Sciences
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-141247 (URN)10.1144/jgs2016-017 (DOI)000395862600013 ()
Available from: 2017-04-18 Created: 2017-04-18 Last updated: 2017-08-10Bibliographically approved
2. Zircon U-Pb geochronology and Nd isotope systematics of the Abas terrane, Yemen: Implications for Neoproterozoic crust reworking events
Open this publication in new window or tab >>Zircon U-Pb geochronology and Nd isotope systematics of the Abas terrane, Yemen: Implications for Neoproterozoic crust reworking events
2015 (English)In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 267, 106-120 p.Article in journal (Refereed) Published
Abstract [en]

High-spatial-resolution secondary-ion mass spectrometry (SIMS) U-Pb zircon ages, whole-rock Nd isotopic and geochemical data are reported for granites and granitic gneisses from a traverse across the Abas terrane, Yemen, a part of the Precambrian basement of southern Arabian Peninsula. SIMS U-Pb dating identifies two magmatic episodes, the first at c. 790-725 Ma represented by granitic gneisses, the second clearly post-tectonic at c. 625-590 Ma. The oldest sample in the post-tectonic group is slightly deformed while younger samples are undeformed indicating that penetrative deformation ceased at c. 625 Ma in the Abas region. Whole-rock(Nd)(t) values between -11 and +0.8, Nd model ages of 1.70-1.13 Ga indicate a significant contribution of evolved continental material in the genesis of the Abas granitoids, unlike most of the juvenile Arabian-Nubian Shield (ANS), although there are few inherited zircons. Secular variation in epsilon(Nd)(t)reflects a change in magma source with increasing juvenile magma and diminishing crustal input during post-tectonic (625-590 Ma) magmatism. The combination of subduction zone chemistry, absence of older rocks, paucity of inherited zircons, evolved Nd isotopic signatures and the I-type characteristics of the samples suggest that assimilation occurred at depth.

Keyword
Precambrian crustal evolution, Arabian Shield, Yemen, U-Pb zircon, Post-tectonic magmatism, Nd isotopes
National Category
Earth and Related Environmental Sciences
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-120897 (URN)10.1016/j.precamres.2015.05.037 (DOI)000359874800006 ()
Available from: 2015-10-09 Created: 2015-09-18 Last updated: 2017-08-10Bibliographically approved
3. Magmatic evolution of the southernmost Arabian shield inferred from integrated zircon U-Pb ages, whole-rock geochemistry, and Nd isotopes
Open this publication in new window or tab >>Magmatic evolution of the southernmost Arabian shield inferred from integrated zircon U-Pb ages, whole-rock geochemistry, and Nd isotopes
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Secondary ion mass spectrometry (SIMS) U-Pb zircon dating of plutonic rocks from the southernmost Arabian Shield identifies the earliest diorite–trondhjemite batholith emplacement at ca. 810 Ma and a second phase of plutonism of similar composition at ca. 685–665 Ma. A major shift in plutonic composition is marked by the emplacement of gneiss domes dated at ca. 770 Ma. Trace element ratios such as high Ba/La, Ba/Nb and Pb/Ce suggest a sub-arc mantle metasomatized by slab-derived fluids. The Th/Ce ratios are also higher than global subducting sediment (GLOSS) and suggesting an incompatible element-enriched mantle source. The distribution in Ta/Nb of the arc related samples beyond the field of mantle-derived magmas suggests a continental influence. From these geochemical characteristics, we conclude that a subducted slab/sediment component, rather than an OIB-like source, is likely. A slab component for the island-arc samples is also corroborated by their typical adakite-like geochemical traits, such as ≥ 60 wt.% SiO2, high Mg# (34–77), high Sr (450–2240 ppm), low HREE and HFSE (0.2–1.1 ppm Yb and 2.6–15.6 ppm Y) and consequently elevated Sr/Y of 117–285, high Ni and Cr contents and negligible Eu-anomalies. All samples gave εNd(t) values of +3.9 to +6.9, overlapping with the western arc terranes of the Arabian Shield, indicating the juvenile nature of the crust in the southernmost Arabian Shield. However, there is a subtle secular variation in εNd(t) with the younger post–tectonic samples being slightly less juvenile. This suggests that the younger plutons were contaminated by minor evolved material.

Keyword
Arabian-Nubian Shield, adakite, subduction, Saudi Arabia, zircon U-Pb, Nd isotopes
National Category
Earth and Related Environmental Sciences
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-145466 (URN)
Funder
Swedish Research Council
Available from: 2017-08-04 Created: 2017-08-04 Last updated: 2017-08-10Bibliographically approved
4. Continental–scale sediment mixing and dispersal across northern Gondwana: Evidence from detrital zircon U–Pb–O–Hf isotope systematics of the Cambrian–Ordovician sandstones of the Arabian Shield
Open this publication in new window or tab >>Continental–scale sediment mixing and dispersal across northern Gondwana: Evidence from detrital zircon U–Pb–O–Hf isotope systematics of the Cambrian–Ordovician sandstones of the Arabian Shield
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The Arabian Shield was a major site of early Neoproterozoic to late Ediacaran juvenile magmatic addition at the northern end of the East African Orogen (EAO) and provides important insights into the amalgamation history of Gondwana. Its voluminous Cambrian–Ordovician cover sandstone, which consistently shows a northerly sediment transport direction, provides constraints on the nature of crustal growth in its presumed source region in the hinterland of Gondwana. However, the provenance of this vast siliciclastic cover sediment in the Saudi Arabian part of the Arabian Shield is poorly understood. We present the first combined in–situ U–Pb, δ18O and Lu–Hf isotopic data of detrital zircons from the Cambrian–Ordovician Saq and Wajid units along the northeastern margin and southern inliers of the Arabian Shield. The U–Pb age spectra reveal prominent age peaks at ~0.7–0.5 Ga and 1.1–0.9 Ga with subordinate peaks at ~2.2–1.7 Ga and 2.7–2.5 Ga. The δ18O secular variation mirrors global compilations with Archean zircons having restricted values of 4.0 to 8.0 ‰ and younger zircons showing larger excursions in δ18O. The 0.7–0.5 Ga age peak shows the largest variation in εHf(t) with half (~47%) of all Neoproterozoic zircons having juvenile εHf(t) (>5) and these are interpreted to have been sourced from the juvenile ANS terranes. The remaining Neoproterozoic zircons with non–juvenile εHf(t) signatures have distal provenance outside of the ANS. As extensive 1.1–0.9 Ga crust is lacking in the vicinity of the ANS, the 1.1–0.9 Ga detrital zircons are interpreted to either be derived from the Kibaran Belt of Central Africa or recycled from sedimentary rocks containing these age components. The large excursions in δ18O in the post–Archean zircons indicate crustal thickening and increased incorporation of supracrustal materials associated with collisional orogenesis. The remarkable similarities in age spectra, isotopic compositions and sediment maturity between our sandstone samples and those from other regions in northern Gondwana, and even modern sediments from rivers that drain the hinterland of Africa, indicate a continental–scale process and demonstrate how supercontinent formation regulates the sedimentary record.

National Category
Earth and Related Environmental Sciences
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-145468 (URN)
Funder
Swedish Research Council
Available from: 2017-08-04 Created: 2017-08-04 Last updated: 2017-08-10Bibliographically approved

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