Maria A. Mange

Petrography of Ordovician and Silurian sediments in the western Irish Caledonides: tracers of a short-lived Ordovician continent-arc collision orogeny and the evolution of the Laurentian Appalachian-Caledonian margin

Abstract Ordovician orogeny affected the Laurentian margin of the Appalachian-Caledonian Belt from the Southern Appalachians to the British Isles and is dated, stratigraphically, as post-uppermost Lower Cambrian and pre-upper Llandovery. Geochronological data favour a short Grampian orogeny from c. 470 to 460 Ma during the late Arenig-mid-Llanvirn, which is supported by the late Arenig-earliest Llanvirn termination of the Laurentian rifted margin carbonate shelf. The likeliest plate tectonic model for the Grampian Orogeny is of a continent-facing oceanic (Laurentia) arc, created as an infant mafic arc during the Middle Cambrian, evolving into an Early Ordovician intermediate-silicic arc with suprasubduction-zone ophiolites. This arc collided with the Laurentian margin and forearc ophiolites were obducted across the margin creating an orogen and accreting the arc to the margin. Continued plate convergence was accommodated by a flip in subduction polarity that terminated late orogenic retrocharriage, and led to the rapid final stages of uplift and unroofing of the orogen and to subduction accretion in the new mid-Ordovician trench. Presented in this paper are definitive new detrital heavy mineral evidence that supports and enhances this model of a short-lived orogenic event involving ophiolite obduction, and the rapid development and unroofing of a Grampian Barrovian metamorphic complex over c. 10 Ma. High-resolution heavy mineral analysis (HRHMA) has been undertaken on most of the Ordovician and Silurian stratigraphical units in the South Mayo Trough in western Ireland. It is shown that ophiolite unroofing began during the Arenig and that, by the early Llanvirn, a Barrovian complex was being eroded. The Killadangan ‘Formation’, of the Clew Bay Complex is confirmed as part of a pre-Grampian early Ordovician accretionary prism by both HRHMA and by quartz-felspar-lithic (QFL) analysis, which show it to have been derived from Precambrian rocks of the Laurentian margin. QFL analysis also shows that the Ordovician provenance of the South Mayo Trough sequence evolves from an undissected arc through a dissected arc to a recycled orogenic detrital pattern, except for the earliest sediments (Letterbrock), which were derived from the transitional continental sediments of the Killadangan accretionary prism, which, in turn, were derived from Laurentia. The problem of timing the Grampian deformation and metamorphism of the Dalradian is now regarded as being solved. It was an Arenig-Llanvirn event lasting c. 10 Ma, which occurred during and is recorded, faithfully, by the detrital heavy mineral assemblage in the conformable Ordovician sequence of the South Mayo Trough. The Miocene evolution of New Guinea is strikingly similar to the Arenig-Llanvirn evolution of the Laurentian margin and was, analogously, the short-lived result of the collision of an arc with the north Australian margin followed by subduction polarity flip.

Chapter 13 Geochemistry of Heavy Minerals

Abstract This contribution provides an insight into the use of mineral chemistry in provenance research, and demonstrates how studies, by using microbeam techniques on several detrital heavy mineral species, benefit from continuing technological advances. Virtually all detrital heavy minerals can now be subjected to sophisticated geoanalytical techniques that can determine their major and trace element compositions and identify their crystal chemistry. Petrogenetic controls impart distinctive elemental signatures to mineral phases in igneous and metamorphic rocks that are preserved in mineral grains eroded from them, and can be used as a genetic tool to help decode their parageneses. Of particular interest are those heavy minerals that are widespread in sediments with compositions that are suitable for routine geochemical analysis. In this review, the geochemistry of garnet, tourmaline, chrome spinel, apatite, pyroxenes and amphiboles and its application to specific geological problems is discussed in detail; brief references are given to minerals that have not been used frequently in provenance studies. It is important to emphasise that a particular species, chosen for geochemistry, is generally associated with other minerals that are also carriers of information. Therefore, mineral chemical data need to be integrated with information from the whole assemblage to ensure that the conclusions are truly comprehensive.

Chapter 6 ‘In Situ’ Dissolution of Heavy Minerals through Extreme Weathering, and the Application of the Surviving Assemblages and their Dissolution Characteristics to Correlation of Dutch and German Silver Sands

Abstract Diverse processes during the sedimentary cycle may generate heavy mineral associations that are devoid of clear signatures of the source region, especially in sediments that experienced unusually severe environmental conditions, and thus their provenance reconstruction becomes problematical. This study provides a new insight into the impact of ‘in situ’ weathering on heavy mineral assemblages, rarely dealt with in recent years, by evaluating the effects of extreme weathering that imparted an unusual bulk and heavy mineral composition to the Tertiary Dutch and German ‘silver sands’ (sands that consist almost exclusively of quartz). Specific findings of our heavy mineral study of these silver sands include: (1) tourmaline can be strongly weathered; (2) the chemical weathering of tourmaline is colour-related and therefore depends probably on its particular chemistry; (3) staurolite is a reliable indicator of the degree of chemical weathering; (4) the effects of extreme chemical weathering on a heavy mineral assemblage differ fundamentally from those of burial diagenesis as, for example, in the total disappearance of apatite; (5) the joint occurrence of fresh and strongly weathered grains (with the same chemical composition) of one heavy mineral species indicates that the degree of chemical weathering is a statistical rather than a fixed parameter; (6) no heavy mineral analysis is reliable if the degree and the effects of in situ weathering are not taken into account; (7) a reliable analysis of extremely weathered sediments requires sand samples of several kilograms. Although regional or local differences in weathering may obscure original heavy mineral compositions and thus impede subdivision of, and correlation between, sedimentary units, understanding the end-products of in situ weathering may also be helpful in drawing stratigraphic boundaries between units with originally comparable heavy mineral compositions, as is proven for the silver sands.

The origin, evolution and provenance of the Northern Belt (Ordovician) of the Southern Uplands Terrane, Scotland: a heavy mineral perspective

The heavy mineral components of Ordovician greywacke turbidites of the Southern Uplands (subduction-accretion prism) Terrane were studied using high-resolution heavy mineral analysis. Diagnostic heavy minerals and the discovery of hitherto unreported species provide a new insight into the interplay of sediment fluxes, tectonic controls and provenance. Each formation is characterized by distinctive heavy mineral suites, indicating that the sediments of successive or laterally-equivalent formations were sourced from different and rapidly-changing lithologies. Appreciable variations were detected even in greywacke packages within an individual formation, indicating that these changes were taking place at a very fast rate. Detritus came from either an eroding Dalradian or a similar Barrovian metamorphic complex and its superjacent obducted ophiolite complex to the northeast, and from oceanic island arcs and collapsing seamounts to the south. The rapid appearance of metamorphic detritus, following a short period of ophiolite stripping, indicates that post-subduction-flip exposure of and erosion into the metamorphic complex was fast, suggesting extensional unroofing. The chemical similarity of detrital garnet with Scottish Grampian garnet may suggest derivation from that complex and, therefore, that the Southern Uplands Terrane has not travelled, sinistrally, more than about 200 km with respect to the Scottish Highlands. The absence of detrital staurolite in the greywackes raises the possibility that stripping either did not reach abundant staurolite levels or that staurolite was a relatively uncommon mineral in the source area. The former explanation is unlikely because, in western Ireland, a similar Dalradian stripping sequence involves the appearance of staurolite in the first arrival of metamorphic detritus. However, possibly, in western Ireland, the Dalradian source was extensionally unroofed to a much deeper structural level than was Scotland in the same time interval. Only the Shinnel Formation shows spatial detrital consistency, indicating that, by Shinnel times, the Dalradian hinterland had achieved a relatively uniform level of erosion. The discovery of lawsonite, as part of a ferroglaucophane-glaucophane blueschist assemblage in the Portpatrick Formation, is important yet enigmatic. Ordovician crosstie-winchite-ferroglaucophane-phengite assemblages are known throughout the northwestern Appalachian/Caledonian belt and may have provided the source for the Portpatrick blueschist-facies detritus. However, a possible source was from the Cadomian blueschist belt of Anglesey, then on the southern margin of the Iapetus Ocean. This is supported by garnets with sodic/calcic amphibole inclusions, unknown in the Dalradian. Perhaps Cadomian ‘terrane fragments’ were rifted from the southern margin of Iapetus to drift towards and collide with Laurentia to shed detritus into the Laurentian margin trench.

Heavy minerals solve structural and stratigraphic problems in Ordovician strata of the western Irish Caledonides

Heavy minerals in Ordovician successions in western Ireland record, in the Upper Arenig Sheeffry Formation, the erosion of an ophiolite/island arc complex. The appearance of staurolite and garnet at a basin-wide horizon in the Lower Llanvirn Upper Derrylea Formation signals the unroofing of the Dalradian metamorphic complex. Parts of the Ordovician sequence on Inishturk and in two small inliers are correlated with the standard sequence with unexpected results. The garnet-, sillimanite-, and staurolite-bearing Letter Formation correlates with the Upper Derrylea Formation and, on Inishturk, heavy minerals in south-younging turbidites reveal a sinistral ramp zone, that places the Sheeffry Formation structurally above and to the south of the younger Derrylea Formation.

Crustal evolution of the South Mayo Trough, western Ireland, based on U–Pb ages and Hf–O isotopes in detrital zircons

Ordovician strata of the South Mayo Trough in western Ireland contain clastic deposits that represent materials eroded from a large and diverse continental area over a time scale that spans much of the Earth’s history. Therefore, it is a useful region to use detrital zircons to construct a continental crustal growth model. Here, we report integrated U–Pb, Lu–Hf and O isotope measurements obtained from in situ analyses of 160 zircons from the South Mayo Trough. U–Pb zircon crystallization ages define three major magmatic episodes of crustal reworking in the Archaean (Lewisian), Mesoproterozoic (Grenville), and Ordovician (Grampian). These data, together with oxygen isotope data and Hf model ages, suggest that crustal growth, recorded in the strata of the South Mayo Trough, started at c. 4 Ga and continued until 1.4 Ga, with two major growth periods at 2.3–2.1 and 2.0–1.5 Ga. We find that the crustal incubation time is decoupled from the duration of supracrustal alteration processes; some zircons with very long crustal incubation times have pristine mantle δ18O signatures suggesting minimal low-temperature surface processing in their source regions. Identifying such zircons is the key for future studies in constructing realistic net continental crustal growth models unaffected by crustal recycling. Supplementary materials: Data tables for U–Pb, Lu–Hf and oxygen isotopes for detrital zircons from South Mayo Trough, as well as plots of values for zircon standards (δ18O for R33, U–Pb ages for 91500 and R33, and 176Hf/177Hf for 91500, GJ-1, and Plešovice) and reverse concordia plots of zircon samples are available at www.geolsoc.org.uk/SUP18543.

Chapter 18 Statistical Analysis of High-Resolution Heavy Mineral Stratigraphic Data from the Ordovician of Western Ireland and its Tectonic Consequences

Abstract High-resolution heavy mineral data from the Ordovician of the South Mayo Trough, western Ireland is subject to a detailed statistical analysis. The aim of this study is to develop new techniques for treating such data thereby providing an independent and objective way to help elucidate the stratigraphy and tectonic history of the region. A new method is presented for plotting normalised scores to emphasise stratigraphic trends of heavy minerals and specific heavy mineral varieties, which are shown to be largely non-normally distributed, with significant variation between formations. A method is developed for testing significance of variation within a formation using non-parametric tests. Techniques are introduced for comparing and correlating formations on the basis of heavy mineral data even when sample sizes are small. The likely causes of correlations between heavy mineral distributions and zircon varieties are discussed, and principal components analysis is used to identify major source regions throughout the life of this basin. This analysis shows that there is a statistically significant change with early deposits dominated by arc-accretionary prism detritus, whereas juvenile metamorphic detritus becomes increasing more important up-section. The South Mayo Trough remained below sea-level during this progressive change in provenance. The generally accepted model for the evolution of this basin is that it represents a fore-arc basin that collided with the Laurentian margin during the Grampian orogeny. The problem of how a hanging wall basin remained below sea-level during and after arc-continent collision is discussed and explained.

Chapter 40 The Provenance of Paste and Temper in Roman Amphorae from the Istrian Peninsula, Croatia

Abstract Roman amphorae, manufactured in the Laecanius workshop in Istria, Croatia, between 10-5 B.C. and 78 A.D, were analysed by integrating archaeological and geological laboratory methods, including thin section petrography, X-ray diffractometry (XRD) and, for the first time, heavy mineral analysis. Characteristics of the fabric allow categorisation of the amphora sherds into fabric groups. Petrography shows that quartz is the dominant clastic component in the sherds while carbonate is common as temper; XRD provided information on firing temperatures that ranged between 750 and 900°C. The amphora sherds contain diverse detrital heavy minerals with generally high epidote and garnet quantities. Occasionally zircon is also important. Garnet/epidote ratios and the presence of diagnostic species (pyroxene, hornblende) show systematic variations that coincide with similar variations in fabric characteristics. Heavy mineral signatures in amphorae produced in other workshops facilitated their distinction from the Laecanius sherds. Comparative heavy mineral analysis of terra rossa found close to the workshop indicate that terra rossa was the major source of the paste. Differences observed in the heavy mineral composition of the amphora sherds and terra rossa are interpreted by the polygenetic nature and spatial heterogeneity of the latter and the mixing of the paste with sandy temper. Modern Adriatic sponge spicules present in the majority of Laecanius amphora sherds and the temper-derived, generally immature, heavy mineral assemblages suggest that the sandy material for the temper was obtained from Adriatic deposits. The heavy mineral technique, as a powerful archaeometric tool, is demonstrated by revealing the provenance of raw materials, and by recognising the compositional difference between the Laecanius amphorae and the sherds from other workshops and the spica sample.

Chapter 16 High-Resolution Heavy Mineral Analysis (HRHMA): A Brief Summary

Abstract High-resolution heavy mineral analysis (HRHMA) is the identification and categorisation of different varieties of individual heavy mineral species, based on the recognition that the majority of rock-forming and accessory minerals form in a diversity of size and habit and are represented by several chemical, structural, colour and optical varieties, controlled primarily by petrogenetic conditions. Because a wide range of lithologies provide detritus to siliciclastic sediments, their heavy mineral assemblages are complex and an individual species may comprise several kinds of varieties. Variables that prove most informative are grain morphology, colour and internal structure. These can be provenance-diagnostic, facies-independent or facies-sensitive. Failure to appreciate such information by grouping the diverse varieties of a particular species into one single mineral category can generate misleading information and often incorrect interpretations, while the full history of a sediment and accurate reconstruction of provenance remains concealed. HRHMA can be easily conducted under the polarising microscope, it is internally consistent and categories can easily be defined and kept without errors. Categorisation and point counting of the varieties of a single species also eliminates problems caused by density-controlled sorting during transport and dissolution processes that affect chemically unstable species.