Chapter 12 Dolerites (1.27–1.25 Ga) and alkaline ultrabasic dykes (c. 1.14 Ga) related to intracratonic rifting

2020 ◽  
Vol 50 (1) ◽  
pp. 315-323 ◽  
Author(s):  
Magnus Ripa ◽  
Michael B. Stephens
Keyword(s):  
Magnetic Susceptibility ◽  
Mantle Plume ◽  
Coastal Area ◽  
Flow Direction ◽  
Tectonic Setting ◽  
Magma Flow ◽  
Depleted Mantle ◽  
Mantle Component ◽  
Back Arc ◽  
Shear Belt

AbstractDoleritic sills, lopoliths and dykes were emplaced into the Paleoproterozoic craton in central Sweden at 1271–1264, 1259–1256 and c. 1247 Ma, a complex temporal zonation occurring in a WSW–ENE direction. The dolerites are subalkaline to alkaline and show predominantly gabbroic, with a trend towards monzogabbroic and quartz monzodioritic, compositions. Positive ɛNd and ɛHf values suggest a significant depleted mantle component in the source volume of the parental magmas. Dyke orientations indicate extension, at least locally, in a northwesterly direction, consistent with a magma flow direction determined using the anisotropy of magnetic susceptibility values. Intracratonic rifting linked to the break-up of the supercontinent Columbia, back-arc extension above a subduction boundary in a westwards-retreating mode or a mantle plume tail above a continental hotspot have all been proposed for the tectonic setting. Renewed intracratonic rifting at c. 1.14 Ga in the coastal area in northeasternmost Sweden resulted in the emplacement of alkaline ultrabasic dykes, including carbonatites (beforsites), silico-carbonatites and lamprophyres, in a north–south direction along an older shear belt. The broader tectonic setting of this extensional event is not known.

scholarly journals Geochemistry and petrology of mafic Proterozoic and Permian dykes on Bornholm, Denmark: Four Episodes of magmatism on the margin of the Baltic Shield

2010 ◽  
Vol 58 ◽  
pp. 35-65
Author(s):  
Paul Martin Holm ◽  
L.E. Pedersen, ◽  
B Højsteen
Keyword(s):  
Mantle Plume ◽  
Mantle Source ◽  
Small Degree ◽  
Spinel Peridotite ◽  
Alkali Basalts ◽  
Enriched Mantle ◽  
Depleted Mantle ◽  
Proterozoic Basement ◽  
The Baltic ◽  
Back Arc

More than 250 dykes cut the mid Proterozoic basement gneisses and granites of Bornholm. Most trend between NNW and NNE, whereas a few trend NE and NW. Field, geochemical and petrological evidence suggest that the dyke intrusions occurred as four distinct events at around 1326 Ma (Kelseaa dyke), 1220 Ma (narrow dykes), 950 Ma (Kaas and Listed dykes), and 300 Ma (NW-trending dykes), respectively. The largest dyke at Kelseaa (60 m wide) and some related dykes are primitive olivine tholeiites, one of which has N-type MORB geochemical features; all are crustally contaminated. The Kelseaa type magmas were derived at shallow depth from a fluid-enriched, relatively depleted, mantle source,but some have a component derived from mantle with residual garnet. They are suggested to have formed in a back-arc environment. The more than 200 narrow dykes are olivine tholeiites (some picritic), alkali basalts, trachybasalts, basanites and a few phonotephrites. The magmas evolved by olivine and olivine + clinopyroxene fractionation. They have trace element characteristics which can be described mainly by mixing of two components: one is a typical OIB-magma (La/Nb < 1, Zr/Nb = 4, Sr/Nd = 16) and rather shallowly derived from spinel peridotite; the other is enriched in Sr and has La/Nb = 1.0 - 1.5, Zr/Nb = 9, Sr/Nd = 30 and was derived at greater depth, probably from a pyroxenitic source. Both sources were probably recycled material in a mantle plume. A few of these dykes are much more enriched in incompatible elements and were derived from garnet peridotite by a small degree of partial melting. The Kaas and Listed dykes (20-40 m) and related dykes are evolved trachybasalts to basaltic trachyandesites. They are most likely related to the Blekinge Dalarne Dolerite Group. The few NW-trending dykes are quartz tholeiites, which were generated by large degrees of rather shallow melting of an enriched mantle source more enriched than the source of the older Bornholm dykes. The source of the NW-trending dykes was probably a very hot mantle plume.

scholarly journals ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS) IN VOLCANIC FORMATIONS: THEORY AND PRELIMINARY RESULTS FROM RECENT VOLCANICS OF BROADER AEGEAN.

2004 ◽  
Vol 36 (3) ◽  
pp. 1308 ◽  
Author(s):  
I. Zananiri ◽  
D. Kondopoulou
Keyword(s):  
Magnetic Susceptibility ◽  
Flow Direction ◽  
Source Region ◽  
Physical Property ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Magnetic Fabric ◽  
Local Flow ◽  
Magma Flow ◽  
Preliminary Results ◽  
Scalar Properties

The anisotropy of magnetic susceptibility (AMS) is a physical property of rocks widely used in petrofabric studies and other applications. It is based on the measurement of low-field magnetic susceptibility in different directions along a sample. From this process several scalar properties arise, defining the magnitude and symmetry of the AMS ellipsoid, along with the magnetic foliation, namely the magnetic fabric. Imaging the sense of magma flow in dykes is an important task for volcanology; the magnetic fabric provides a fast and accurate way to infer this flow direction. Moreover, the AMS technique can be used in order to distinguish sills and dykes, a task that is almost impossible by using only field observations. Finally in the case of lava flows, the method can be applied to define the local flow conditions and to indicate the position of the "paleo" source region. However, this technique is quite new in Greece. Some preliminary results from volcanic formations of continental Greece and Southern Aegean are presented (Aegina, Almopia, Elatia, Gavra, Kos, Patmos, Samos, Samothraki and Santorini).

A Cretaceous back-arc basin in the Coast Belt of the northern Canadian Cordillera: evidence from geochemical and neodymium isotope characteristics of the Kluane metamorphic assemblage, southwest Yukon

2001 ◽  
Vol 38 (1) ◽  
pp. 91-103 ◽  
Author(s):  
Jochen E Mezger ◽  
Robert A Creaser ◽  
Philippe Erdmer ◽  
Stephen T Johnston
Keyword(s):  
Late Jurassic ◽  
Metamorphic Rock ◽  
Tectonic Setting ◽  
Source Region ◽  
Canadian Cordillera ◽  
Metasedimentary Rocks ◽  
Depleted Mantle ◽  
Metamorphic Assemblage ◽  
Continental Source ◽  
Back Arc

The Coast Belt of the northern Cordillera in Canada is the locus of the boundary between accreted and ancient North American margin rocks. The largest exposure of metasedimentary rocks in the Coast Belt is the Kluane metamorphic assemblage (KMA), a northwest-striking belt 160 km long of graphitic mica–quartz schist and gneiss with minor interfoliated olivine serpentinite. The KMA does not appear to correlate with other sedimentary or metamorphic rock assemblages in the Canadian Cordillera. To determine its tectonic setting and protolith provenance, we analyzed trace element, rare earth elements, and neodymium isotope compositions of the KMA, of the adjacent pericratonic Aishihik metamorphic suite (AMS) of the Yukon–Tanana terrane, and of adjacent slates of the Dezadeash Formation (DF), filling a Late Jurassic – Early Cretaceous flysch basin. The εNd(0) values of analyzed KMA samples range from –1.4 to –5.6 and depleted mantle model ages (TDM) range from 1.16 to 1.45 Ga. KMA samples are intermediate between more evolved AMS samples (average εNd(0) –25, TDM = 2.6 Ga) and more juvenile DF samples (εNd(0) = +1.9, TDM = 0.95 Ga). The intermediate characteristics of the KMA samples cannot be linked to a known source region and are interpreted to reflect homogeneous mixing from predominantly juvenile and minor evolved sedimentary sources. A compatible tectonic setting is a back-arc basin within influence of a continental source. Eastward subduction of the KMA beneath ancient North America collapsed the back-arc basin by latest Cretaceous time.

Permian post-collisional basic magmatism from Corsica

2020 ◽  
Author(s):  
Andrea Boscaini ◽  
Andrea Marzoli ◽  
Joshua H.F.L. Davies ◽  
Massimo Chiaradia ◽  
Hervé Bertrand
Keyword(s):  
Tectonic Setting ◽  
Western Alps ◽  
Dyke Swarm ◽  
Variscan Orogeny ◽  
Calc Alkaline ◽  
Depleted Mantle ◽  
Extensional Tectonic ◽  
Basic Magmatism ◽  
Southern Central ◽  
Mantle Component

&lt;p&gt;Post-Variscan early Permian magmatism is widespread in Corsica with mafic dykes emplaced during the extensional tectonic phase which followed the Variscan orogeny. This study focuses on a mafic dyke swarm intruded in the region of Ajaccio (Corsica, France). New U-Pb zircon geochronological data show that these intrusions were emplaced at ca. 282 Ma. Most Ajaccio dykes have a calc-alkaline affinity, while a few dykes show tholeiitic affinity resembling N-MORB basalts. Calc-alkaline to tholeiitic dykes are characterized by enriched to depleted Sr-Nd-Pb isotopic compositions, respectively. We interpret these data as evidence that an enriched mantle source, which was likely formed during Variscan subduction, sourced the calc-alkaline suite, while a depleted mantle component dominates the source of the tholeiitic suite. Notably, coeval Permian mafic intrusive bodies from throughout Corsica and from the Southern, Central and Western Alps display similar ages and geochemical features to the Ajaccio dyke swarm. This indicates that a widespread Permian magmatic province developed in a post-orogenic extensional tectonic setting at the margin of the former Variscan belt&lt;/p&gt;

Evidence for an 18O-depleted mantle plume from contrasting 18O/16O ratios of back-arc lavas from the Manus Basin and Mariana Trough

2000 ◽  
Vol 176 (2) ◽  
pp. 171-183 ◽  
Author(s):  
Colin G. Macpherson ◽  
David R. Hilton ◽  
David P. Mattey ◽  
John M. Sinton
Keyword(s):  
Mantle Plume ◽  
Depleted Mantle ◽  
Manus Basin ◽  
Back Arc

On the rock magnetic properties and petrofabric (AMS) analyses of trachytic, phonolitic, basaltic dikes and conesheets emplaced on the Miocene Tejeda caldera complex, on Gran Canary Island, Spain.

2021 ◽  
Author(s):  
Emilio Herrero-Bervera ◽  
Manuel Calvo Rathert
Keyword(s):  
Magnetic Properties ◽  
Magnetic Susceptibility ◽  
Canary Islands ◽  
Flow Direction ◽  
Hysteresis Loops ◽  
Radial Symmetry ◽  
Magnetic Mineral ◽  
Magma Flow ◽  
Gran Canaria ◽  
Magnetic Fabrics

&lt;p&gt;The Miocene Tejeda Complex on Gran Canaria (Canary Islands) is characterized by more than 500 trachytic and phonolitic conesheets, dikes, hypabyssal syenite stocks and subordinate radial dikes from a 20-km diameter intrusive complex in the volcaniclastic fill of the Miocene Tejeda caldera (20 x 35 km) on Gran Canaria, Canary Islands. The dikes intruded concentrically around a central axis or radial symmetry and dip uniformly an average of~41 degrees toward the center.We have conducted a pilot study of magnetic properties as well as Anisotropy of Magnetic Susceptibility (AMS) on a variety of dikes (trachytic and phonolitic and basaltic in composition) to investigate the possibility of obtaining petrofabrics results that would allow us to test the origin of the formation of the Tejeda conesheets that most likely resulted from deformation processes due to resurgent doming initiated by recurrent replenishment of a flat&amp;#160; laccolith-like magma chamber. The current ideas indicate that the formation of the cone-shaped fractures were originated by a magma supply exceeding the volume that could be compensated for by up-doming of the overlying caldera fill. Thus far, our AMS results indicate that all the ten intrusives studied despite their different lithologies are susceptible of carrying a measurable magnetic signal. Low-field magnetic susceptibility vs temperature (k-T 28-700&lt;sup&gt;o&lt;/sup&gt;C) experiments have identified mainly one primary magnetic mineral phase namely stoichiometric magnetite, Curie temperature of 585&lt;sup&gt;o&lt;/sup&gt;C,&amp;#160; SIRM, hysteresis loops and back-field were performed and yielded a series of secondary magnetic mineral present as well and corroborated by FORC&amp;#8217;s results. The petrofabric in the intrusive bodies results show coherent flow azimuths regardless of their time of emplacement. Three main types of magnetic fabrics, (i.e. A to C) were found. Fabric type A (plane Kmax-Kint parallel to the dike plane) represents magma flow direction within the intrusives and is the dominant fabric (~60% of all the intrusives) studied thus far. The Kmax axis inclinations show that about 70% of the intrusives were fed by inclined vertical magma fluxes (inclinations greater than 30&lt;sup&gt;o&lt;/sup&gt;), and the rest of them (~30%) by horizontal to sub-horizontal magma fluxes. Vertical magma flow means inclined magma injection inside fractures, and become more probable as the source is therefore located very close to the origin of the caldera&lt;/p&gt;

Stratigraphy, structure and geochemistry of Archaean supracrustal rocks from Oqaatsut and Naajaat Qaqqaat, north-east Disko Bugt, West Greenland

1999 ◽  
pp. 65-78
Author(s):  
Henrik Rasmussen ◽  
Lars Frimodt Pedersen
Keyword(s):  
Tectonic Setting ◽  
Trace Element Geochemistry ◽  
Element Geochemistry ◽  
West Greenland ◽  
North West ◽  
North East ◽  
Metavolcanic Rocks ◽  
Arc Setting ◽  
Area North ◽  
Back Arc

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Rasmussen, H., & Frimodt Pedersen, L. (1999). Stratigraphy, structure and geochemistry of Archaean supracrustal rocks from Oqaatsut and Naajaat Qaqqaat, north-east Disko Bugt, West Greenland. Geology of Greenland Survey Bulletin, 181, 65-78. https://doi.org/10.34194/ggub.v181.5114 _______________ Two Archaean supracrustal sequences in the area north-east of Disko Bugt, c. 1950 and c. 800 m in thickness, are dominated by pelitic and semipelitic mica schists, interlayered with basic metavolcanic rocks. A polymict conglomerate occurs locally at the base of one of the sequences. One of the supracrustal sequences has undergone four phases of deformation; the other three phases. In both sequences an early phase, now represented by isoclinal folds, was followed by north-west-directed thrusting. A penetrative deformation represented by upright to steeply inclined folds is only recognised in one of the sequences. Steep, brittle N–S and NW–SE striking faults transect all rock units including late stage dolerites and lamprophyres. Investigation of major- and trace-element geochemistry based on discrimination diagrams for tectonic setting suggests that both metasediments and metavolcanic rocks were deposited in an environment similar to a modern back-arc setting.

scholarly journals Geochemical Variation of Miocene Basalts within Shikoku Basin: Magma Source Compositions and Geodynamic Implications

Minerals ◽  
2020 ◽  
Vol 11 (1) ◽  
pp. 25
Author(s):  
Shuang-Shuang Chen ◽  
Tong Hou ◽  
Jia-Qi Liu ◽  
Zhao-Chong Zhang
Keyword(s):  
Japan Sea ◽  
Isotopic Signature ◽  
Magma Source ◽  
Isotopic Compositions ◽  
Enriched Mantle ◽  
Shikoku Basin ◽  
Depleted Mantle ◽  
Parece Vela Basin ◽  
Back Arc ◽  
Ocean Ridge

Shikoku Basin is unique as being located within a trench-ridge-trench triple junction. Here, we report mineral compositions, major, trace-element, and Sr-Nd-Pb isotopic compositions of bulk-rocks from Sites C0012 (>18.9 Ma) and 1173 (13–15 Ma) of the Shikoku Basin. Samples from Sites C0012 and 1173 are tholeiitic in composition and display relative depletion in light rare earth elements (REEs) and enrichment in heavy REEs, generally similar to normal mid-ocean ridge basalts (N-MORB). Specifically, Site C0012 samples display more pronounced positive anomalies in Rb, Ba, K, Pb and Sr, and negative anomalies in Th, U, Nb, and Ta, as well as negative Nb relative to La and Th. Site 1173 basalts have relatively uniform Sr-Nd-Pb isotopic compositions, close to the end member of depleted mantle, while Site C0012 samples show slightly enriched Sr-Nd-Pb isotopic signature, indicating a possible involvement of enriched mantle 1 (EM1) and EM2 sources, which could be attributed to the metasomatism of the fluids released from the dehydrated subduction slab, but with the little involvement of subducted slab-derived sedimentary component. Additionally, the Shikoku Basin record the formation of the back-arc basin was a mantle conversion process from an island arc to a typical MORB. The formation of the Shikoku Basin is different from that of the adjacent Japan Sea and Parece Vela Basin, mainly in terms of the metasomatized subduction-related components, the nature of mantle source, and partial melting processes.

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