No description available.

No description available.

No description available.

No description available.

The cores of most Cycladic Islands are formed as consequences of early Paleogene high-pressure subduction processes of the African plate beneath Europe, and Miocene extensional exhumation of the subduction trench. The island of Thera (Santorini) resides in the Hellenic Volcanic Arc, and is dominated by Quaternary eruptive volcanic material atop a pre-volcanic basement. The position of the island has led to debate as to the nature of the pre-volcanic basement, with connections drawn to either the Cycladic Blueschist Unit (CBU) or the Phyllite-Quartzite Unit (PQU). Field observations, which document a top-to-SSE detachment, in conjunction with geochronological techniques have been applied to assess the tectonostratigraphy of the pre-volcanic basement. The results resolve the pre-volcanic basement as belonging to the CBU which has been juxtaposed against Sub-Pelogonian marble by the SSE directed detachment. This firmly establishes Thera within the Cycladic realm, documents Miocene deformation, and changes the geologic map of the Hellenides.

No description available.

Short-lived isotope systems can be utilized to track differentiation processes that had occurred during Earth’s early history. Both the 182Hf-182W and 146Sm-142Nd systems are sensitive to silicate fractionation events due to differing parent-daughter incompatibilities. The 182Hf-182W system is also affected by metal-silicate fractionation events due to the siderophile nature of W. An increasing number of mantle-derived rocks, mainly formed during the Archean (4.0-2.5 Ga), have presented variable anomalies in the daughter products of both systems, indicating their sources contain isotope signatures established shortly after the formation of the Earth. Some Archean Fe-rich primitive magmas known as ferropicrites, have been suggested to derive from mantle domains that differentiated after the crystallization of a Hadean (>4.0 Ga) magma ocean. In order to investigate the potential involvement of a Hadean source in the petrogenesis of Archean ferropicrites, we have studied the Nd and W isotopic compositions of rocks from the Pulpwood-Playter Harbour sequence in the Wawa subprovince, Ontario, Canada. This sequence is composed of ferropicritic intrusive rocks and lavas in association with tholeiitic mafic lavas. A 147Sm-143Nd isochron including all lithologies yields an age of 2681±51 Ma (MSWD =6.6) with an initial 143Nd of +2.5. This Nd initial isotopic composition indicates the rocks were derived from a long-term incompatible-element depleted mantle source. Both the ferropicritic and the tholeiitic rocks plot on the same isochron, suggesting they derived from the same mantle source, despite their different geochemical compositions. Negative 142Nd anomalies compared to the Nd terrestrial standard were found in the majority of the rocks studied here, yielding an average μ142Nd value of -2.0±3.9. Although not resolvable from the terrestrial standard given iii the current analytical precision, the fact that almost all analysed samples exhibit negative μ142Nd values could suggest the influence of a Hadean source in their formation. If this is the case, a single early silicate fractionation event occurring between 4.56 and 4.47 Ga could explain both the ε143Nd and μ142Nd values obtained for the studied rocks. Alternatively, the involvement of eclogitic material, with a Hadean basaltic protolith, interacting with Archean peridotitic mantle could explain the Nd isotopic composition of the ferropicrites, but this would not account for the identical isotopic composition of the tholeiites – thus rendering a garnet pyroxenite source improbable. One intrusive ferropicritic sample yielded a resolvable 182W excess of +14.1 ±6.7 ppm. If this excess 182W is characteristic of the Pulpwood-Playter Harbour sequence, it indicates the decoupling of 182Hf-182W and 146Sm-142Nd systems. This decoupling could be explained by early metal-silicate fractionation recorded in the Hf-W systematics of these rocks or the contribution of Fe-rich meteoritic material into the source of ferropicrites.

The first billion years of our planet’s history is almost devoid of geological records and this scarcity of Eoarchean/Hadean rocks and minerals greatly limits our understanding of how and when the first crust formed on Earth. The Nuvvuagittuq Greenstone Belt (NGB), located in the Hudson Bay terrane of the Northeastern Superior Province, may host the oldest preserved rocks on Earth. It is locally intruded in its southwestern corner by rare 3.76 Ga trondhjemite bands which impose a minimum age for the NGB, but its dominant lithology, a mafic cummingtonite-amphibolite called the Ujaraaluk unit, displays isotopic evidence suggesting it may represent a rare remnant of Hadean mafic crust as old as 4.3 Ga. However, this proposed Hadean age for the NGB has been heavily debated for more than a decade. As potentially the only remnant of crust formed within the first 500 million years of Earth’s history, the NGB could have important implications on our knowledge of the first terrestrial crust. In order to impose tighter geochronological constraints on the NGB, here we present U-Pb zircon and ¹⁴⁷Sm-¹⁴³Nd whole rock data for gneissic gabbro sills that intrude the Ujaraaluk unit as well as U-Pb data for zircons from intruding and surrounding granitoids. A new strategy of sampling for the gneissic gabbros targeted the most evolved plagioclase-rich zones and amphibole-rich cumulative rocks to better constrain their age of magmatic differentiation. The most evolved parts of the sills were also sampled because they are the most likely to contain igneous zircon or baddeleyite that could constrain their crystallization age. Zircons from two compositionally evolved gneissic gabbros yielded U-Pb ages between 2.7 and 2.6 Ga consistent with the timing of Neoarchean metamorphism in the region. A plagioclase-rich layer found within the gabbro sills yielded zircons defining a U-Pb Concordia upper intercept age of 2789 Ma, but their texture and Th/U ratios are more consistent with recrystallization of zircon subsequent to the breakdown of an older Zr-bearing phase and therefore do not constrain the age of emplacement of the sills. However, a ¹⁴⁷Sm-¹⁴³Nd isochron for the gneissic gabbros, including the newly identified plagioclase-rich evolved zone and hornblende-rich cumulative rock, yielded an isochron age of 4151 ± 290 (MSWD = 9, n = 6) interpreted as the timing of magmatic differentiation of the sills. This 4.1 Ga age thus strongly supports the previously proposed Hadean age for the NGB. New zircon U-Pb data reported here for plutonic trondhjemites found in the central and eastern parts surrounding the NGB suggests that the extent of the ~3.8 Ga Eoarchean felsic magmatism is greater than previously thought. New zircon trace element and oxygen isotope data for a series of granitoids surrounding and locally intruding the NGB previously dated at 3.76 Ga, 3.66 Ga, 3.51 Ga and 3.35 Ga provide a better understanding of the petrogenetic processes responsible for early felsic crust production. Zircons from the 3.76 Ga, 3.66 Ga and 3.35 Ga granitoids are characterized by rare earth element trends typical of unaltered igneous zircons. However, zircons from the 3.51 Ga magmatic event display unusual rare earth element patterns, with a striking positive Eu-anomaly, suggesting that they may have experienced some type of post-magmatic alteration. Zircon δ¹⁸O values appear to have slightly increased over time with the zircons from the oldest 3.76 Ga trondhjemites displaying mean δ¹⁸O values within the mantle zircon field and the younger Paleoarchean granitoids progressively deviating from the mantle zircon field to reach a mean zircon δ¹⁸O value of 6.58‰ at 3.35 Ga. This suggests that the 3.76 Ga trondhjemites were derived from an unaltered crustal source, whereas the ≤3.66 Ga granitoids were derived from a supracrustal source that had experienced some degree of low-temperature hydrothermal alteration. This trend of mantle-like zircon δ¹⁸O values preserved in the first evolved crust, which deviate towards higher zircon δ¹⁸O values in successive felsic magmatic events, has also been observed in other Hadean and Eoarchean terranes indicating that similar processes may have operated on a global scale during the production and evolution of early continental crust.

In the central Aegean of Greece, metamorphic rocks have been brought to the surface along multiple detachment faults producing a series of core complexes. The detachment faults have been grouped into several detachment systems including the North and West Cycladic Detachment Systems. The timing and magnitude of displacement along individual detachment faults is variable, the faults exhumed footwall rocks with a range of metamorphic grades from different crustal depths. Geochronology (zircon (U-Th)/He and white mica ⁴⁰Ar/³⁹Ar analyses) was conducted to investigate the timing of activity along a paired detachment system exposed in the bedrock of Mt. Hymittos, Attic Peninsula, Greece. Taken together the geochronometers indicate the detachments of Hymittos were active from the late Oligocene to the late Miocene with both faults active during the middle Miocene. The ductile-to-brittle deformation of the structures, top-SSW kinematics, morphology of the massif, and metamorphic grade indicate that these detachments are extensions of the West Cycladic Detachment System. Additionally, the overlap in timing between the structures indicates that paired detachment systems can be coeval. Coeval displacement along separate detachment branches has important implications for strain partitioning within the crust and the development of detachment systems. Detrital zircon U-Pb geochronology was conducted on samples from exposures of the West Cycladic Detachment System on the Attic Peninsula. The results are compared to unpublished data from the Attic Peninsula and the Western Cyclades including Santorini. The analyses confirm the correlation of the footwall rocks of Hymittos to the Cycladic Blueschist Unit as well as evaluate several paleogeographic correlations for the Cycladic Blueschist Unit. The samples herein record significant recycling of Variscan zircon from the Cycladic Basement, strongly suggesting the proto-Cycladic Blueschist Unit was deposited atop the Cycladic Basement. The Pindos Zone of the Hellenides is the most likely paleogeographic equivalent to the Cycladic Blueschist Unit. Strengthening this correlation gives additional insight into the position of the proto-Cycladic Blueschist Unit which is important for understanding both the movement of Hellenic terranes through time, and the pre-subduction relationship(s) between the terranes that today comprise the Attic-Cycladic Complex. Finally, electron backscatter diffraction analysis was utilized to examine the mechanisms of deformation in a suite of calcitic and dolomitic marble mylonites from the detachments of Hymittos. This was used to shed light on the conditions under which these ductile-then-brittle structures deformed suggesting that dolomite deformed under lower to sub-greenschist facies conditions may be surprisingly weak when deforming by grain size sensitive processes. Dolomite is a major rock forming mineral and comprises significant parts of carbonate dominated terranes and (meta)sedimentary successions. The apparent weakness of dolomite reported herein suggests that under greenschist and sub-greenschist facies conditions dolomite may accommodate significant strain not predicted by experiments or commonly reported from the field. Together this thesis provides insight into detachment fault architecture, deformation processes and the pre-subduction dynamics and structural position of the geologic units exposed within the Attic-Cycladic Complex.

I present in-situ analysis of U-Pb systematics in baddeleyite and whitlockite grains from a suite of Martian shergottites. 9 baddeleyite grains (5 from basaltic shergottite NWA 2986 and 4 from olivine-phyric shergottite RBT 04262) were analyzed by LA-MC-ICP-MS. Despite low uranium and radiogenic lead concentrations , maximum ages could be determined for both samples: 187 ± 50 to 1236 ± 430 for NWA 2986 and 100 ± 9 to 526 ± 48 for RBT 04262. The same analytical procedures were used for whitlockites in NWA 2986, ALHA 77005, EETA 79001, NWA 2646 and LAR 06319. As with the baddeleyite analyses, maximum ages were calculated. These ages ranged from 110 ± 1 for LAR 06319 to 561 ± 185 for NWA 2646. These results, which are consistent with previous analyses, mean that the ~ 4 Ga age determined from Pb-Pb analyses cannot time the igneous crystallization of these meteorites.