The modern science of stratigraphy is founded on a nineteenth- century empirical base – the lithostratigraphy and biostratigraphy of basin-fill successions. This stratigraphic record comprises the most complete data set available for reconstructing the tectonic and climatic history of Earth. However, it has taken two hundred years of evolution of concepts and methods for the science to evolve from what Ernest Rutherford scornfully termed “stamp collecting” to a modern dynamic science characterized by an array of refined methods for documenting geological rates and processes. Major developments in the evolution of the science of stratigraphy include the growth of an ever-more precise geological time scale, the birth of sedimentology and basin-analysis methods, the influence of plate tectonics and, most importantly, the development, since the late 1970s, of the concepts of sequence stratigraphy. Refinements in these concepts have required the integration of all pre-existing data and methods into a modern, multidisciplinary approach, as exemplified by the current drive to apply the retrodicted history of Earth’s orbital behaviour to the construction of a high-precision ‘astrochronological’ time scale back to at least the Mesozoic record. At its core, stratigraphy, like much of geology, is a fieldbased science. The field context of a stratigraphic sample or succession remains the most important starting point for any advanced mapping, analytical or modeling work.
Crystal LaFlamme, Christopher R. M. McFarlane and David Corrigan
The Repulse Bay block (RBb) of the southern Melville Peninsula, Nunavut, lies within the Rae craton and exposes a large (50,000 km2) area of middle to lower crust. The block is composed of ca. 2.86 Ga and 2.73–2.71 Ga tonalite-trondhjemite-granodiorite (TTG) and granitic gneiss that was derived from an older 3.25 and 3.10 Ga crustal substrate. This period of crustal generation was followed by the emplacement of ca. 2.69–2.66 Ga enderbite, charnockite, and granitoid intrusions with entrained websterite xenoliths. These voluminous batholith-scale bodies (dehydrated and hydrated intrusions), and the associated websterite xenoliths, have similar whole rock geochemical properties, including fractionated light rare earth element (LREE)–heavy (H)REE whole rock patterns and negative Nb, Ti, and Ta anomalies. Dehydrated intrusions and websterite xenoliths also contain similar mineralogy (two pyroxene, biotite, interstitial amphibole) and similar pyroxene trace element compositions. Based on geochemical and mineralogical properties, the two lithologies are interpreted to be related by fractional crystallization, and to be the product of a magmatic cumulate processes. Reworking of the crust in a ca. 2.72 Ga subduction zone setting was followed by ca. 2.69 Ga upwelling of the asthenospheric mantle and the intrusion of massif-type granitoid plutons. Based on a dramatic increase in FeO, Zr, Hf, and LREE content of the most evolved granitoid components from the 2.69–2.66 Ga cumulate intrusion, we propose that those granitoid plutons were in part derived from a metasomatized mantle source enriched by fluids from the subducting oceanic slab that underwent further hybridization (via assimilation) with the crust. Large-scale, mantle-derived Neoarchean sanukitoid-type magmatism played a role in the development of a depleted lower crust and residual sub-continental lithospheric mantle, a crucial element in the preservation of the RBb.
M. B. Petrie, J. A. Gilotti, W. C. McClelland, C. van Staal and S. J. Isard
The St. Cyr area near Quiet Lake hosts well-preserved to variably retrogressed eclogite found as sub-metre to hundreds of metre-long lenses within quartzofeldspathic schist in southcentral Yukon, Canada. The St. Cyr klippe consists of structurally imbricated, polydeformed and polymetamorphosed units of continental arc crust and ultramafic–mafic rocks. Eclogite-bearing quartzofeldspathic schist forms thrust slices in a 30 km long by 6 km wide, northwest-striking outcrop belt. The schist unit comprises metasedimentary and felsic intrusive rocks that are intercalated on the metre to tens of metres scale. Ultramafic rocks, serpentinite and associated greenschist-facies metagabbro form imbricated tectonic slices within the eclogite- bearing quartzofeldspathic unit, which led to a previously held hypothesis that eclogite was exhumed within a tectonic mélange. The presence of phengite and Permian zircon crystallized under eclogite-facies metamorphic conditions in the quartzofeldspathic host rocks indicate that the eclogite was metamorphosed in situ together with the schist as a coherent unit that was part of the continental arc crust of the Yukon–Tanana terrane, rather than a mélange associated with the subduction of oceanic crust of the Slide Mountain terrane. Petrological, geochemical, geochronological and structural similarities link St. Cyr eclogite to other high-pressure localities within Yukon, indicating the high-pressure assemblages form a larger lithotectonic unit within the Yukon–Tanana terrane.
Incompatible elements and isotopic ratios identify three endmember
mantle components in oceanic island basalt (OIB); EM1, EM2, and
HIMU. We estimate compatible to mildly incompatible transition metal
abundance trends (Ni, Co, Fe, Cu, Cr, V, Mn, Sc, and Zn) in
‘primitive’ basalt suites (Mg# = Mg/(Mg + 0.9*Fe) atomic = 0.72)
from 12 end-member oceanic islands by regressing metals against
Fe/Mg ratios in sample suites, and solving for concentrations at
Mg/Fe = 1 (Mg# = 0.72). Using the transition metal estimates,
exploratory statistics reveal that islands ‘group’ based on mantle
component type even when La/Yb ratios are used to compensate metal
concentrations for percentage melting. Higher chalcophile Zn (and
Pb, earlier work) in EM1 and EM2 compared to HIMU, and higher Cr
(3+) and Sc in HIMU relative to EM1, support views that HIMU
represents subductionprocessed ocean floor basalt. Incompatible
elements, ratios and isotopes indicate that EM1 is Archean, EM2 is
Proterozoic or younger, and both are related to sediment subduction.
As found with incompatible elements, EM1 and EM2 show similar
‘compatible’ element concentrations, but lower (multivalence) Cr, Fe
and Mn in EM1 could indirectly reflect increasing oxidation of
subducted sediment between the Archean and Proterozoic.
Alternatively, changes in subduction processes that yielded peak
continental formation in the Neoarchean, and craton-suturing in the
Paleoproterozoic may account for EM1–EM2 differences. EM1 shows
similar or lower Cr, Ni and Co compared to HIMU and EM2 suggesting
that economic viability of layered intrusions, which have extreme
EM1-like signatures, is unrelated to high metals in EM1 mantle
sources, but that high % melting appears important. Because
core-concentrated transition metals correlate with mantle component
type, lithospheric recycling apparently controls their
concentrations in OIB and core-mantle interaction may be