The Iberian Massif represents a well-exposed cross section of continental crust affected by Hercynian oblique continent-continent collision during the Late Devonian to Carboniferous. The internal Central Iberian Zone (CIZ) of the Iberian Massif is occupied by thick metasedimentary sequences of Late Proterozoic to Paleozoic age, which show an overprint of lower greenschist to upper amphibolite facies Hercynian regional metamorphism. Basal parts of the terrigenous metasedimentary sequence were affected by partial melting associated with widespread generation and intrusion of Hercynian granitoids.
Tectonostratigraphic relationships suggest that turbiditic Late Proterozoic / Cambrian (LPC) metasediments belong to a Pan-African / Cadomian distal synorogenic flysch sequence deposited at an active continental margin, whereas Late Proterozoic (LP), Ordovician (ORD) and S ilurian (S IL) metasediments were deposited on stable continental cru st in passive tectonic environments.
The Nd isotope evolution recorded in the LP (eNd[i] -6.3), LPC (-2.3), ORD (-9.5) and SIL (-9.0) metasediments is not compatible with large-scale involvement of ancient continental crust as source of the detritus (e.g. Archaean / Early Proterozoic crust) nor with derivation from a single crustal segment of Middle to Late Proterozoic age. Instead elevated eNd[i] values and lower TCR crustal residence ages of the LPC metasediments (TCR 1.3 Ga VS TCR > 1.6 Ga in the LP, ORD and SIL metasediments) signify important additions of juvenile material with less enriched Nd isotope signatures to the LPC detritus. The large geochemical variation of the LPC metaturbidites is attributed to several processes: mechanical sorting during paleoweathering, transport, and sedimentation, which caused separation of quartz and clay minerals into different grain size fractions, resulting in strong inverse correlation of most major and trace elements with the SiO2 content ('quartz dilution effect'), 'heavy mineral effects', seen through either largely variable or constant abundance of elements such as Na2O, P205 and Zr, which are contained in particular heavy minerals or detrital minerals other than quartz and clay-minerals (e.g. feldspar), and diagenesis, burial and Hercynian regional metamorphism, which obliterated primary sedimentary fabrics through recrystallization, and caused small scale redistribution of LILE and partial resetting of the Rb-Sr isotope systematics.
Effects of these processes must be considered when using sediment geochemistry for identification of provenance and tectonic environment of deposition. Conventional geochemical classification using normalized multi-element diagrams, bivariate plots, and triangular diagrams, only partially succeeds in separating geochemical effects of provenance from those of depositional and post-depositional processes. Element ratio diagrams that combine immobile trace elements characteristic for felsic crustal sources (e.g. Th, LREE) with trace elements characteristic for mafic/mantle sources (e.g. Sc, V, Cr) more effectively relate geochemical signatures to source rock compositions, and hence to provenance and depositional tectonic environment. In the proposed binary trace element ratio diagrams, the LPC metasediments and the group of LP, ORD and SIL metasediments consistently occupy separate fields, in agreement with interpretations based on tectonostratigraphic and Nd isotope geochemical data. It is concluded that geochemistry of terrigenous sediments in general can be used for classification of provenance and tectonic environment of deposition, if due account is given to the modifying effects of depositional and post-depositional processes.
The inferred presence of at least 30 % Pan-African / Cadomian juvenile components in the LPC metaturbidites contrasts with the scarcity of juvenile components related to younger Paleozoic orogenic activity (e.g. Caledonian) in Paleozoic sediments of the Iberian Massif. This results in a bimodal rather than continuous distribution of Nd isotopic signatures in the Late Proterozoic / Paleozoic metasedimentary sequence and its basement in the Iberian Massif, which constitute the crustal protoliths for Hercynian granitic partial melts. Hercynian granitoids in the N Portuguese CIZ include ~350-310 Ma weakly to strongly peraluminous, alkaline syntectonic granitoids and ~320-280 Ma calcalkaline to alkaline post-tectonic granitoids.
The N Portuguese syntectonic Hercynian granitoids represent early partial melts that were related to crustal thickening. The Hercynian syntectonic granitoids comprise the subgroups of syn-Dl+2 gneiss-granites, syn-D3 two-mica granites and syn-D3 biotite-rich granites. The limited geochemical variation of the entirely crustal-derived syntectonic granitoids reflects primarily the apparent compositional heterogeneity of the crustal protoliths and suggests negligible fractional crystallization. The apparent heterogeneity is related to differences in conditions of partial melting and subsequent segregation of partial melts from melt-depleted source material ('restite'). The initial Nd isotope composition of all subgroups of syntectonic Hercynian granitoids appears to show a bimodal distribution with culminations at eNd[i]-3.0 to -5.0 and -6.0 to -8.0, representing two discrete crustal protoliths and corresponding to the Hercynian Nd isotopic signatures of Late volumes of early post-tectonic mantle-derived gabbroic magmas and large volumes of crustalderived partial melts with (isotope) geochemical characteristics similar to that of the syntectonic granitoids. Partial melting in the anatectic crustal source region was enhanced upon intrusion of the early post-tectonic mantle-derived melts and this gave rise to large magma chambers in which the composition of the early hybrid granitoids was further modified by combined processes of assimilation of cru stal melts and fractionation of clinopyroxene- , amphibole- and plagioclase-rich cumulates (AFC processes).
The N Portuguese syntectonic and post-tectonic Hercynian granitoids are thus predominantly derived from recycling of heterogeneous metasedimentary and / or metaigneous c ru stal sou rces of M id dle Protero zoic to Paleo zoic age , and record merely limited crustal growth in terms of addition of j uvenile mantle-derived material to the cru st. The apparent relation between initial Nd isotope composition of the Hercynian granitoids and location relative to trends of main D3 Hercynian structures suggest a structural control on depth of, and hence crustal protoliths affected by Hercynian partial melting. It is therefore concluded that crustal recycling is an efficient and important process, and that the potential for Nd isotope systematics to identify crustal protoliths involved in crustal reworking and partial melting in a particular region depends on the rate of succession of earlier orogenic periods and amounts of juvenile material delivered to the continental crust and the terrigenous sedimentary record during or shortly after these periods.
N Portuguese mantle-derived (ultra)mafic units of Paleozoic age indicate that lithospheric mantle sources, after a long-term depleted history, underwent significant enrichment in inco mpatib le trace ele ments le s s than ~ 200 M a b efore extracti on of the (ultra)mafic units from the sub-Iberian mantle reservoir. The heterogeneous incompatible trace element enrichments are tentatively related to either the latest stages of subduction of oceanic lithosphere prior to the Pan-African / Cadomian orogeny or to extension during Cambrian / Early Ordovician times. The considered (ultra)mafic units do not allow identification of similar enrichments of the sub-Iberian mantle reservoir related to subduction of oceanic lithosphere prior to the Hercynian orogeny.
The geochemistry of Late Proterozoic / Paleozoic terrigenous metasediments, Hercynian granitoids and Paleozoic mantle-derived units discussed in this thesis illustrate the profound importance of the ~700-550 Ma Pan-African / Cadomian orogeny in the cru stal evolution of the Iberian Massif. Crustal growth related to this orogenic period is recorded by large volumes of mantle-derived material added to pre-existing crust of Middle Proterozoic age and it was apparently accompanied by significant geochemical modification the lithospheric mantle.
Proterozoic / Paleozoic metasediments containing and lacking Pan African / Cadomian juvenile components, respectively. The post-tectonic Hercynian batholithic complexes show a succession of intrusions that include the subgroups of early post-tectonic late- / post-D3 mafic and intermediate intrusives, late- / post-D3 monzogranites, and post-D3 biotite granites. The subgroup of late- / post-D3 mafic and intermediate intrusives and their microgranular or 'magmatic' enclaves define a calcalkaline differentiation trend, and the enclaves and host rocks consistently show geochemical and Nd isotope disequilibrium.
The different subgroups of successive posttectonic intrusives are characterized by large (isotope) geochemical variations (e.g. eNd[i] between +0.9 and -7.2), and a systematic decrease of eNd[i] values with progressive geochemical differentiation is observed both within and between the different subgroups of post-tectonic intrusives. |
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