From static to dynamic provenance analysis—Sedimentary petrology upgraded

    From static to dynamic provenance analysis – sedimentary petrology upgraded Eduardo Garzanti PII: DOI: Reference:

S0037-0738(15)00160-8 doi: 10.1016/j.sedgeo.2015.07.010 SEDGEO 4888

To appear in:

Sedimentary Geology

Received date: Revised date: Accepted date:

14 May 2015 9 July 2015 22 July 2015

Please cite this article as: Garzanti, Eduardo, From static to dynamic provenance analysis – sedimentary petrology upgraded, Sedimentary Geology (2015), doi: 10.1016/j.sedgeo.2015.07.010

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From static to dynamic provenance analysis –

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sedimentary petrology upgraded

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Eduardo Garzanti

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Laboratory for Provenance Studies, Department of Earth and Environmental Sciences, Università di Milano-Bicocca, 20126 Milano, Italy

E-mail address: [email protected]

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ABSTRACT The classical approach to sandstone petrology, established in the golden years of plate tectonics and based on the axiom that "detrital modes of sandstone suites primarily reflect the different tectonic

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settings of provenance terranes”, has represented a benchmark for decades. The composition of

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sand and sandstone, however, simply provides us with a distorted image of the lithological structure

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of source terranes, and gives us little clue whether they are allochthonous or authochthonous, orogenic or anorogenic, young or old. What we may able to see reflected in detrital modes is the nature of source terranes (continental, arc, oceanic) and the tectono-stratigraphic level reached by

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erosion in space and time. The proposed new approach to the petrology of sand and sandstone: 1)

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starts with a simple classification scheme circulated since the 1960s, which is purely descriptive, objective and free of ill-defined ambiguous terms; 2) focuses on the nature and tectono-stratigraphic level of source terranes. Further steps are essential to upgrade provenance analysis. Acquiring

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knowledge from modern settings is needed to properly identify and wherever possible correct for

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physical and chemical processes introducing environmental and diagenetic bias, and thus address

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nature's complexities with adequate conceptual tools. Equally important is the integration of multiple techniques, ideally including bulk-sediment, multi-mineral and single-mineral methods. Bulk-sediment petrography remains the fundamental approach that allows us to capture the most

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precious source of direct provenance information, represented by the mineralogy and texture of rock fragments. Bulk-sediment geochemistry, applicable also to silt and clay carried in suspension, is a superior method to check for hydraulic sorting, chemical weathering and fertility of detrital minerals in different sediment sources. Detrital geochronology, thermochronology and isotope geochemistry reveal the diverse time structures of source rocks, and have become necessary complementary techniques in modern provenance analysis. Inferences on geodynamic processes need independent geological information and come last, but if tackled properly they can lead us much farther than the standard label obtained by using triangular diagrams uncritically as if they were infallible oracles. Keywords: Sandstone petrology; Unroofing trends; Dickinson model; Bulk-sediment methods; Multi-mineral methods; Single-mineral methods.

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“Once you have accepted a theory and used it as a tool in your thinking, it is extraordinarily difficult to notice its flaws.” Kahneman, 2011 p.277

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1. Introduction

The origin of sedimentary petrology dates back to the late 19th century, with the invention of thinsection petrography by H.C. Sorby. Provenance studies flourished in the first half of the 20th century, when P.D. Krynine - inspired in Moscow by the ideas of his teacher M.S. Shvetsov -

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became the first strong advocate of tectonic control on sandstone composition. A feverish activity in sedimentary petrology followed the first classification schemes proposed in the late 1940s (Krynine,

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1948; Pettijohn, 1954), and new classifications continued to proliferate through the 1950s and 1960s (McBride, 1963; Dott, 1964; Folk, 1980). Virtually all of these were projected onto triangular diagrams, which force us to consider three parameters only at a time (quartz, feldspars, rock

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fragments, or in other cases micas or "clay matrix"). The underlying conceptual schemes were not

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based on a thorough investigation of modern settings (Suttner, 1974), and did not benefit yet from the breakthrough of the plate-tectonic revolution. Major progress came with the work of Dickinson

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(1970), who a few years after Gazzi (1966) established operational rules to improve the reproducibility of detrital modes and showed how to connect them with paleogeodynamic scenarios in a seemingly straightforward univocal way (Dickinson and Suczek, 1979). The new paradigm

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hinged on the axiom that "detrital modes of sandstone suites primarily reflect the different tectonic settings of provenance terranes” (Dickinson, 1985 p.333). A parallel, somewhat milder statement applied to geochemical composition - echoing Crook (1974) - is Bhatia's (1983 p. 611) “close correlation exists between the geochemical composition of sandstones and tectonic settings of sedimentary basins”. Such claims reflect the enthusiasm of those years, when plate-tectonic theory finally made geologists aware of the fundamental controls of tectonic processes, and seemed able to provide a fresh new explanation for each geological phenomenon. The power of suggestion was strong, and the enchantment persisted through time. Even though other schools refused to adopt the same radical attitude and focused more on compositional modifications during erosion, transport and deposition (e.g., Suttner et al., 1981), sedimentary petrology has remained nailed to that vision for three decades, confirming that “knowledge is in the end based on acknowledgment” (Wittgenstein, 1974 #378). In provenance analysis, as in other fields, we may either trust a simplified model based on what is believed to be essential, or surrender to nature‟s baffling complexities and interpret each case as the

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unique result of multiple competing causes, giving up the quest for a general theory (Paola and Leeder, 2011). To overcome such trade-off, and preserve the benefits that theory offers as a basis for interpretation (Weltje, 2012), we need to acknowledge that the lithological characteristics of

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parent rocks as inferred from detrital modes of a daughter sandstone cannot be used blindly as a

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proxy for plate-tectonic setting. The path leading from a handful of sand to a geodynamic scenario has long been known to be winding and fraught with difficulties (Basu, 1985; Johnsson, 1993). And

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yet, if we make the most of their great potential, then traditional petrographic methods can provide us with a simple and very effective means to identify the nature and tectono-stratigraphy of source terranes, and thus with an unexcelled key to track their erosional evolution through space and time.

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Whenever basic tools such as the optical microscope are left aside in favour of advanced efficient machines that produce a great deal of numbers on a very minor and possibly non-representative

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fraction of the sediment, the ultimate risk is to find ourselves facing a sea of data without a suitable vessel to sail. The rapid development of technological devices should not make us feel that culture

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is superfluous, and thus induce us to throw away the traditional keys to understanding.

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2. Complexity versus simplicity in provenance analysis French in origin and borrowed from the arts, the term “provenance” refers to the succession of

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passages experienced by a work of art before reaching a museum or a private collection. Provenance analysis, an art by itself, is a thorny business (Zuffa, 1987; Weltje and von Eynatten, 2004). The primary signals imparted by source-rock lithologies undergo diverse physical and

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chemical modifications during the sedimentary cycle, both before (“environmental bias”) and after deposition (“diagenetic bias”). Physical and chemical processes produce fundamentally different effects on sediment composition. Whereas the former complicate matters chiefly by distorting the primary signal and can be generally understood and corrected for, the latter may destroy a major and generally unassessable part of the signal, which is thus virtually impossible to restore. Mechanical break-down affects selectively the least durable unconsolidated sedimentary rock fragments (e.g., shale), but not significantly most other framework grains (McBride and Picard, 1987; Garzanti et al., 2015a). Hydrodynamic sorting by size, density and shape controls intrasample and intersample variability, with effects that can be identified and neutralized even in the extreme case of placer lags (Garzanti et al., 2009; Weltje et al., 2015). Chemical weathering, negligible in cold or arid climates (Nesbitt and Young, 1996; Potter et al., 2001; Garzanti et al., 2003), fosters development of thick soil profiles in hot humid climate, with prominent effects (Garzanti et al., 2013a). Corrosion features of detrital minerals offer important

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clues on their relative resistance to weathering (Velbel, 2007; Andò et al., 2012), but tell us the state of what is preserved without helping much to assess what was destroyed. Even quartz and zircon grains suffer from chemical dissolution, and there is no way to recalculate accurately what is gone

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from what remains.

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Most drastic are the effects of post-depositional leaching, because the available time for chemical reactions is much longer, and intrastratal temperatures significantly higher. Many common heavy

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minerals are chemically unstable during burial diagenesis, and are dissolved extensively or even completely in sandstones older than the Plio-Pleistocene. Pyroxene goes first, and next in succession amphibole, epidote, titanite, staurolite and garnet; below 3-4 km only zircon, tourmaline,

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rutile, apatite and Cr spinel stand good chances to survive (Morton and Hallsworth, 2007). In most ancient sandstones, heavy-mineral suites consequently represent only the meager durable residue of the much richer and more varied original detrital population. Post-depositional dissolution of main

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framework grains may also be extensive, producing “diagenetic quartzarenites” in extreme cases (McBride, 1985). Burial diagenesis is not only an efficient serial killer. During lithification, cement

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and other authigenic phases may grow in abundance, distorting the orginal composition further,

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which represents a particularly serious problem for the interpretation of chemical analyses in the absence of careful petrographic observations. Moreover, chemical effects may be impossible to tell

1967; Dott, 2003).

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from those of recycling, a widespread phenomenon very hard to quantify for sandstone suites (Blatt,

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2.1. Starting from the simplicity of a descriptive classification

To bring simplicity in the foreground, we must start anew from the solid basis of accurate description, and get rid of the encrustations of the past, including historical names such as "graywacke" or "arkose" still inexplicably in vogue in geological teaching and literature. The brute term graywacke, referring undecidedly at the same time to colour (gray), texture ("matrix-rich", poorly sorted, angular), composition (lithic-rich but containing quartz and feldspar as well), sedimentary process (typically turbidite) and even age (typically Paleozoic), has generated permanent confusion since the early days of its introduction (Murchison, 1854 p.359; Folk, 1954; Krynine, 1956; Cummins, 1962; Dott, 1964; Dickinson, 1970). Following Folk (1980 p.128), a "graywacke" is nothing else than "a very hard, ugly, dirty, dark rock that you can’t tell much about in field". Arkose is at best an unnecessary ambiguous synonym of feldspar-rich sandstone (Huckenholz, 1963). The attachment to such fantasy names with the idea that "genesis must and does permeate our classification" (Pettijohn, 1948, p.113) retarded analytical investigation by

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involuntarily promoting mythic thinking (sensu Dickinson, 2003). Claiming that genesis should be the basis for understanding is pretending to know beforehand what we want to investigate. Whenever we use "judgments as principles of judgment" we soon find ourselves "going round in a

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circle" (Wittgenstein, 1974 #124, #191).

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The simplest nomenclature, introduced by Crook (1960 p.425) and endorsed by Dickinson (1970 p.697) and Weltje (2006 fig. 2), characterizes sands and sandstones by an adjective reflecting the

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relative abundance of their three main components (quartz, feldspars, lithic fragments). Accessory phyllosilicates or heavy minerals, generally less abundant and markedly affected by hydraulicsorting processes because of their platy shape or high density, are neglected to reduce complexity

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and come closer to a transport-invariant measure of sediment composition (Weltje, 2004). The scheme proposed here (Fig. 1) is based on QFL detrital modes obtained by the Gazzi-Dickinson

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point-counting method (Ingersoll al., 1984; Zuffa, 1985), modified to record in full detail all encountered rock fragments, which is where the richest and most direct provenance information lies (Suttner and Basu 1985; Garzanti and Vezzoli, 2003). To define aphanitic lithic fragments (L pole)

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- including all volcanic, ultramafic, metamorphic and sedimentary types and thus also chert,

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limestone and dolostone - we use the same 62.5 m cut off (conventional boundary between silt and sand) as Dickinson (1970), whereas Gazzi (1996) proposed originally a 30 m cut off (boundary

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between particles with cohesive and frictional behaviour). The main components, considered only where exceeding 10%QFL, are listed in order of abundance (e.g., in a quartzo-feldspatho-lithic sand L > F > Q > 10%QFL, in a feldspatho-lithic sand L > F > 10%QFL > Q). Because there are as many

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rock-fragment types as rocks, useful additional information can be included freely in the definition by a suitable modifier reflecting the most common rock-fragment group (e.g., volcaniclastic, carbonaticlastic, metamorphiclastic, ultramaficlastic; Ingersoll, 1983). A ready objection is that the limited number of variables allowed by graphical displays (three by the equilateral triangle, four by the equilateral tetrahedron projected on the plane) cannot render justice to the richness of detrital suites, including a great variety of rock fragments, feldspars and accessory minerals. To visualize a virtually unlimited number of variables in bi-dimensional space we may recur then to the biplot (Gabriel, 1971). This very efficient statistical/graphical tool helps us not only to discriminate among sample groups, but also to understand the mutual relationships among variables. Parameters other than compositions and of heterogeneous nature can be plotted as well (e.g., grain size in m, distance from the source in km, time in Ma), but the chosen scales of measurement have an impact on the graphical display. The biplot uses the logarithmic transformation, and thus requires that data are all positive (i.e., all zero values in the entry table should be replaced by a suitably small positive number). The principal disadvantage is that each

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addition of a variable or of a sample modifies the biplot. Provenance fields of reference, therefore, cannot be defined as done commonly with Cartesian or triangular diagrams.

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2.2. Unraveling complexity by exploring modern Earth

Our chance of making correct provenance diagnoses becomes small when geological age blurs our

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landmarks, and the complexity of natural systems confounds us in a labyrinth of possibilities. The only hope to carry out the task successfully is to learn as much as we can from modern cases, where we do not have to worry about the superposed effects of diagenesis and full geological and

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geomorphological information on source terranes and sediment-routing systems is available, thus providing suitable conditions in which each control on sediment composition can be identified and

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individually quantified (Ingersoll, 1990; Le Pera and Critelli, 1997). The study of modern settings helps us to perceive the complexities of source-to-sink dispersal paths, and the difficulties involved in the reconstruction of ancient landscapes (Allen, 2008; Hinderer, 2012). Only by comparison with

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the modern can we understand and correct for hydraulic-sorting effects, evaluate the importance of

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weathering and recycling, realize the uncertainties and potential pitfalls nested in our thinking, and

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ultimately avoid moving in circle in the attempt to confirm our prejudices. 3. The integrated approach to provenance diagnosis

The best strategy to tackle complexity is to combine a diversified set of analytical methods and

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provenance tracers (Najman, 2006). Different techniques provide distinct points of view, from which disparate details of the general picture can be revealed. Only by the painstaking careful integration of such diverse complementary pieces of information can we hope to get a glimpse of the entire landscape. 3.1. Implicitly chosen grain-size windows

Terrigenous sediments range from clay to boulders, spanning many orders of magnitude (i.e., from few microns to several meters). There is hardly a single method applicable to such a huge spectrum of grain sizes. While choosing a certain analytical approach we must remain fully aware that we have consequently restricted our focus to the limited - sometimes very limited - part of the grainsize spectrum that can be investigated by the chosen technique. The harder we work on that size window, the easier it becomes to forget that we are in the meantime disregarding the rest.

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Most techniques work well on sand only. Gravel must be generally tackled in the field, with limited means. Muds and mudstones, representing the largest part of the sedimentary record (Blatt, 1985), are hardly dealt with by optical methods or single-grain techniques, and geochemistry, X-ray diffraction or

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Raman spectroscopy are generally used to monitor suspended load, which makes up the bulk of fluvial

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sediment transport (Bangs-Rooney and Basu, 1994; Andò et al., 2011; Bouchez et al., 2011). Geochemistry is the only flexible technique that can be equally employed on clay, silt, sand and

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granules, and therefore on most components of the sediment flux (von Eynatten et al., 2012). Bulk petrography has a more limited range, being well suited to sand-sized sediments only. Information quality and analytical precision decrease quickly from very fine sand to very coarse silt, and little can

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be done on cohesive mud. Very-coarse sand and fine gravel are suited only for the detailed analysis of rock fragments, because thin sections are too small to contain a representative sample of such coarse

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clasts. Pebbles, cobbles and boulders have to be studied one by one, which may provide precious information on the lithologies that yield specific detrital minerals, on the orogenic versus anorogenic geochemical affinity of volcanic parent rocks, or on the age and exhumation histories of plutonic and

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metamorphic source terranes (Bluck et al., 2006; Dunkl et al., 2009; Spalla et al., 2009). Where

2013b; Limonta et al., 2014).

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sandstone clasts occur, within-clast point-counting is essential in the study of recycling (Garzanti et al.,

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Heavy-mineral analyses can be performed down to medium silt size under the optical microscope, and almost in the entire silt range with the help of Raman spectroscopy (Andò and Garzanti, 2014). Singlemineral techniques are applied to a more limited part of the grain-size spectrum (typically very coarse silt to lower medium sand; Lawrence et al., 2011; von Eynatten and Dunkl, 2012). Focusing on one

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single detrital-mineral population (e.g., zircons), representing a minimal part of a minor part of the total sediment flux (e.g., sand bedload), brings on the risk of ignoring the bulk of what lies out there in nature (Weltje and von Eynatten, 2012).

3.2. Bulk-sediment methods

Because of the limitations implicit in any technique, in most provenance studies we end up to analyze sets of sediment samples that may be scarcely representative of the entire spectrum of grain sizes contained in the total sediment flux. To avoid making things worse by analyzing sub-samples not even representative of the sample from which they were extracted, bulk-sample approaches should be the starting point of any provenance investigation. Taking into account the largest possible fraction of total transported sediment is a pre-requisite to avoid the pitfalls of source-rock

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fertility, and to calculate sufficiently accurate sediment budgets from which average erosion rates can be derived. Traditional petrography is by far the most straightforward, cheap and efficient means to determine

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the mineralogical and textural parameters characterizing the sand-sized sediment under

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examination. Rich information on parent rocks is revealed directly by the detailed study of rock fragments, which provide us with a firm basis for provenance diagnosis. In particular, the rigorous

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definition and systematic determination of metamorphic rank for each counted metapelite, metapsammite and metabasite rock fragment allows us to estimate the average metamorphic grade of parent rocks (MI index of Garzanti and Vezzoli, 2003), and hence the crustal level reached by

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erosion in source areas (Garzanti et al., 2006, 2010b). Although the recent tendency is to rely more and more on sophisticated instruments for the collection of numerical datasheets, the use of

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valuable data-acquisition machinery does not automatically lead to valuable science, and basic examination of thin sections under the microscope remains very necessary. One microscope and thin sections are often enough to look at a sand-sized sediment or sedimentary

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rock and unveil many of its secrets, even though the famed Kryninism "give me one thin section

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and I'll give you the story of the Appalachian geosyncline" (Bates and Griffiths, 1971) is received as an obvious overstatement. There are quite a few important provenance features that petrography

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cannot capture. Without the help of detrital geochronology nothing is seen of the time structures of source terranes, and we cannot guess whether they are young or old, allochthonous or autochtonous, and thus orogenic or anorogenic. There is no ready way to discriminate detritus derived from sections of continental crust exposed along a rifted margin or incorporated tectonically in the

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external belt of an orogen, from neometamorphic or paleometamorphic sources within the same orogen, from a neometamorphic axial belt or an old cratonic crust, from recent or old ophiolitic sutures, from active neovolcanic and inactive paleovolcanic arcs or anorogenic volcanic fields. Geodynamic setting, therefore, cannot be inferred directly from detrital modes of sand and sandstone. Bulk-sediment geochemistry represents a suitable complementary approach, and the wide spectrum of chemical elements and their different behavior provide invaluable information in sediment-generation studies. The abundance of ultradense species (e.g., zircon) can be calculated approximately from the concentration of elements hosted principally in that mineral (e.g., Zr, Hf), which makes geochemistry a most efficient tool to assess the fertility of different sediment sources (Dickinson, 2008) as well as hydraulic-sorting effects leading to the anomalous concentration of such minerals in different samples or size classes (Garzanti et al., 2010a, 2011). Most chemical elements, however, are hosted in significant proportions in diverse detrital minerals, which blurs the geochemical fingerprint of specific

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sources while detritus gets progressively homogenized during downstream transport in higher-order rivers to the sea (Ingersoll, 1990; Garzanti et al., 2014). Moreover, those primary signals may be effaced by orders-of-magnitude stronger hydrodynamic effects, and bulk-sediment geochemistry is

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thus generally revealed as a rather blunt tool for provenance diagnosis. A happy exception is

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represented by elements hosted preferentially in mafic and ultramafic rocks (i.e., Cr, Ni and to a lesser extent Mg; von Eynatten et al., 2003), which carry a signal commonly strong enough to survive

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downcurrent homogenization and environmental bias (Amorosi, 2012; Garzanti et al., 2012). Sediment geochemistry is also widely used to infer the extent of chemical weathering (Nesbitt and Young, 1982; Price and Velbel, 2003; Borges et al., 2008; Shao et al., 2012), although weathering, hydrodynamic,

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grain size and provenance effects must be detangled carefully to avoid misinterpretations (Bloemsma et al., 2012; Garzanti and Resentini, 2015; von Eynatten et al., 2015). The extensive dissolution of

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unstable minerals and massive precipitation of cements and authigenic material during burial diagenesis may considerably reduce the usefulness of bulk-sediment geochemistry in the provenance study of ancient sandstones. 87

Sr/86Sr,

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Nd/144Nd) provide important information on crustal evolution

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Isotope ratios (e.g.,

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(Goldstein and Jacobsen, 1988) and are generally less sensitive to environmental and diagenetic bias. The relative sediment contribution from two sources with very distinct isotopic fingerprints

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can be assessed accurately, but where detritus is derived from multiple tectonic domains with overlapping signatures, as commonly is the case in orogenic settings or large river systems, then single isotopic ratios give equivocal responses and can serve us at best to check provenance estimates made independently with other methods (Clift et al., 2002; Padoan et al., 2011).

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Moreover, we must decipher the relative role played by the detrital minerals that control the isotopic budget of bedload and suspended load in fluvial systems (Garçon et al., 2014).

3.3. Multi-mineral methods

A complete panorama of the great potential of heavy-mineral studies is provided in the monumental book by Mange and Wright (2007) and in the detailed updated review by Morton (2012). The investigation of multi-mineral suites reveals crucial provenance information for paleotectonic reconstructions (e.g., Dewey, 2005), especially if coupled with petrographic observations. Two fundamental aspects for the correct description and interpretation of heavy-mineral assemblages, whether they are analyzed optically or by more advanced techniques (e.g., microprobe, QemScan), are however overlooked frequently.

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The first aspect concerns the practical need to select a specific grain-size window for analysis wherever the presence of detrital grains with great size differences makes laboratory procedures troublesome (Mange and Maurer, 1992). Under the wrong implicit assumption that grain-size

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classes represent transport-invariant subpopulations, and with the illusion that narrowing the size-

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window increases consistency whereas in fact it is prone to maximize bias, several authors recommended to analyze a single class not more than 1 or even only 0.5wide (125-250 μm,

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Carver, 1971; 63-125 μm, Morton, 1985; 90-125 μm, Bateman and Catt, 2007). Because highdensity minerals settle at the same velocity of - and hence are deposited together with - much coarser low-density or platy minerals, the former concentrate in the fine tail of any sorted sediment,

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the latter in the coarse tail (Rubey, 1933; Garzanti et al., 2008). The different grain-size classes of a sorted sediment, therefore, have notably different composition, and bulk-sample or multiple-

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window analyses represent the only correct options to estimate the percentages of detrital minerals accurately. Provenance diagnoses or stratigraphic correlations cannot be made any more accurate at either local or regional scale by considering a homogeneously narrow size range for a texturally

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inhomogeneous suite of terrigenous rocks. To minimize analytical bias, well-sorted sands should be

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analyzed in bulk, whereas the widest possible size-window centered about the mean should be chosen for poorly-sorted sediments (Garzanti et al., 2009). Point-counting techniques are highly

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recommended for heavy-mineral analyses carried out on bulk-samples or wide grain-size windows. This is because the discrepancy between the real volume percentages and the number percentages as determined by grain counting increases with the increasing width of the grain-size window analyzed, and volume percentages are systematically overestimated for denser minerals that are

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smaller than settling-equivalent lower-density minerals (Galehouse, 1971). Image analysis represents a useful complementary technique to tackle such operational problems. The second aspect concerns the need to estimate not only the relative abundance of diverse heavy minerals but also their absolute abundance (i.e., their concentration in the bulk sample; Garzanti and Andò, 2007a). The concentration (and not just the spectrum) of heavy minerals in a sedimentary deposit depends on the composition of parent rocks, and increases by more than an order of magnitude during progressive unroofing of denser rocks found at deeper-seated crustal levels (Garzanti et al., 2006). Equally drastic modifications of their concentration (as well as of their spectrum) may occur by selective entrainment of low-density grains in the depositional environment (Garzanti et al., 2010a), or by selective leaching of unstable species during diagenesis (Gazzi, 1965; Andò et al., 2012). The concentration of heavy minerals (as well as their spectrum) is therefore per sè crucial in provenance interpretations and in the correct assessment of recycling, hydraulic and diagenetic processes. The distortive fertility effect related to the different potential of different rock

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types to generate heavy minerals must always be taken into full account in the interpretation of heavy-mineral suites, which tend to document aberrantly a limited number of sources (e.g., mafic igneous and metamorphic rocks), whereas several others are barely recorded (limestone, chert,

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shale, granite). In the absence of significant weathering and diagenesis, mafic rocks may thus

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impose their mark on the heavy-mineral spectrum even where their outcrops are sparse (figure 1 in

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Garzanti and Andò, 2007a). 3.4. Single-mineral methods

An excellent comprehensive synthesis of the advanced techniques and target minerals used in

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modern provenance analysis is provided in von Eynatten and Dunkl (2012). Minerochemical methods can be applied to any detrital species displaying significant compositional variability,

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whereas geochronological and thermochronological methods are limited to the narrower range of suitable minerals containing unstable isotopes. Fractionation by physical or chemical processes is less fastidious in the case of single-mineral approaches, because size, shape, density and chemical

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durability vary less within a single-mineral population than within the entire detrital population.

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After the advent of detrital geochronology and thermochronology we can now investigate the diverse time structures of source terranes (Vermeesch et al., 2009), a powerful complement to the

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traditional petrographic or geochemical approaches that provide information on the lithological structure of parent rocks only. Zircon, widespread in recycled sands and diagenized sandstones because of its durability, is the most commonly targeted mineral. U-Pb age spectra of detrital

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zircons reflect the crystallization ages of exposed magmatic and metamorphic rocks, whereas their fission-track age and length distributions - if not reset after deposition - are related to exhumation mode and timing in parent-rock domains. Zircon-rich felsic rocks, however, will show up much better than zircon-poor ones even though their outcrop area or erosion rate are limited, and other source rocks will not show up at all, including basalt, serpentinite, carbonate or chert. Because of zircon durability, age spectra may remain unchanged through successive recycling episodes and consequently homogeneous in time and space, ceasing to be useful provenance tracers (Garzanti et al., 2013c). And even zircon populations may be fractionated by hydrodynamic processes (Lawrence et al., 2011). But the most insidious peril of single-mineral approaches is that the researcher may end up to believe that “what he sees is all there is” (Kahneman, 2011 p.85). Provenance inferences obtained from a single mineral cannot be extrapolated to the bulk sediment unless its fertility in all potential sources is known accurately (Moecher and Samson, 2006; Malusà et al., 2015), which is hardly ever the case. Zircon has exceptionally useful properties, but its average content in sediments is only 2

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grains out of 10,000 (roughly corresponding to 200 ppm of Zr in the upper continental crust; Taylor and McLennan, 1995). By focusing on zircon exclusively we shall miss information from the remaining 99.98% of the sample. Exploring a very minor portion of the explorable may not turn out

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to be the clever strategy. While a powerful beam of light illuminates a small detail, the rest of the

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picture remains in darkness.

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4. The plate-tectonic paradigm

The plate-tectonic revolution of the 1960s and 1970s offered a new elegant conceptual framework

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and opened up novel ways to explore the history of continents and oceans. Past geodynamic scenarios could now be unveiled by trace-element signatures of igneous rocks (Pearce and Cann,

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1973; Winchester and Floyd, 1977), and the intuition that the same could be done with the compositional signature of terrigenous sediments inspired the outstanding contribution of the Dickinson school in the 1970s and 1980s, as well as various geochemical studies (Bhatia, 1983;

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Roser and Korsch, 1986). The hope that the mark of ancient plate-tectonic settings could be testified

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univocally by, and thus inferred unequivocally from detrital modes of sandstones gave rise to great scientific interest and a boom of sandstone-petrology studies in successive years. By emphasizing

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plate-tectonic control, however, the new model pushed to the background the compositional modifications induced by physical and chemical processes during the sedimentary cycle, and all details of sediment generation. Moreover, the geodynamic framework of reference was conceptually simplified to the point that disillusion inevitably followed, and in successive decades

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petrographic methods were progressively abandoned in favor of more sophisticated approaches in the quest of those magical solutions that traditional techniques seemed unable to provide. Criticism of the model came early, but was largely limited to specific exceptions (Mack, 1984) rather than on more substantial flaws, such as the oversimplification of orogenic domains and the disregard of rift-related settings, anorogenic volcanism and ophiolitic sources. Carbonate rock fragments, widespread in all but the wettest monsoonal and equatorial climates (Zuffa, 1985, 1987), were largely neglected. Chert was considered typical of Subduction Complex provenance, which is contradicted by studies of modern depositional systems (Garzanti et al., 2002a, 2013b). But above all, sandstone petrography alone cannot tell us the age of igneous or metamorphic source rocks, or whether they are allochthonous or autochtonous. Orogenic and anorogenic provenances, therefore, can hardly be discriminated (Fig. 2), and geodynamic setting cannot be univocally inferred from detrital modes of sandstones.

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Another major problem with the Dickinson model is that it was built mostly on ancient sandstone suites, more than 80% and 90% of clastic units considered respectively in Dickinson and Suczek (1979) and Dickinson et al. (1983) being older than the Neogene. Because ancient geodynamic

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settings are assumed rather than known, the model is partly based on circular reasoning. The

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rigorous analysis by Weltje (2006), following that of Molinaroli et al. (1991), shows that the identified Continental Block, Magmatic Arc and Recycled Orogen provenances can be

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distinguished by Dickinson‟s plots with a success rate of only 64% to 78%. These considerations indicate clearly that, in spite of its fame and faithful application in so many provenance studies, the model cannot be used blindly as an oracle (and even less as a classification).

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The mistake, however, does not reside in the model but rather in its uncritical use. Models are not meant to be infallible, but just conceptual tools built because they are easier to handle than nature

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itself (Borges, 1960). As for spherical cows (Paola and Leeder, 2011), we should appreciate the theoretical simplification rather than taking them as gospel truth. Dickinson himself (1985 p. 351; 1988 p.9) advised to use ternary diagrams as graphical devices to illustrate mixing of detritus from

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diverse end-member sources, rather than rigidly as discrimination oracles of universal validity. Such

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a flexible creative approach allows us to describe sediment mixing in space and time (Dickinson, 1985 p.352-354), thus moving forward from the standard choice of a static paleogeographic

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scenario to the methodical definition of unroofing trends reflecting the dynamic evolution of diverse crustal sources (Fig. 3).

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4.1. From static to dynamic sedimentary petrology The seminal article by Dickinson and Suczek (1979) opened the door for plate-tectonics to enter the field of sedimentology, and plate-tectonic control on sediment composition has remained the dominant paradigm for the subsequent decades. The model works excellently for magmatic arcs, the geodynamic setting most thoroughly investigated in successive years (Marsaglia and Ingersoll, 1992 and references therein). Magmatic arcs are relatively easy to model as commonly isolated simple systems. The classical unroofing trend from volcaniclastic to plutoniclastic detritus associated with progressive deepening of erosion into the arc massif, defined originally in the Great Valley forearc sequence (Dickinson and Rich, 1972; Ingersoll, 1983), confirmed subsequently its validity even in more complex orogenic settings (Garzanti et al., 1996). Such an application of triangular diagrams to predict changing detrital modes during unroofing of the source terrane (e.g., figure 3 in Dickinson, 1985) indicated the way to tackle other anorogenic and orogenic provenances with the same dynamic approach (Garzanti et al., 2001, 2002b, 2004, 2010b). Considering the evolution of source areas in time provides us with a richer perspective in provenance analysis, leading us to

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examine the stratigraphic record carefully and to look for the trace of eroding source terranes within changing paleogeographic scenarios (e.g., table 2 in Garzanti et al., 2012). To make a step forward, however, some of the tenets on which the classic paradigm is based need to

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be abandoned. First of all, the illusion that sediment composition is controlled strictly by plate-

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tectonic setting, whereas in reality sediment composition simply provides us with a distorted image of the lithological structure of source terranes. Second, the illusion that sediments generated in

they simply do not (Fig. 2; Fig. 3).

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5. Tectono-stratigraphic levels and unroofing trends

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different settings should plot diligently in separate places within a QFL diagram, whereas in reality

The idea that sediment composition is primarily controlled by the tectono-stratigraphic level

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undergoing erosion in the source area was first expressed clearly by Krynine (1948), who envisaged the continental crust as consisting of three layers: sediments on top, metamorphic rocks and veins

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next, and plutonic igneous rocks at depth. With increasing amounts of tectonic activity (and/or time) these successively deeper layers are brought to the surface and act as a source for sediments,

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yielding in succession "quartzite", lithic-rich "graywacke" and feldspar-rich "arkose" (Folk, 1980 p.108). A similar partition is found in Continental Block provenance, subdivided by Dickinson and

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Suczek (1979 p.2175) into Craton Interior subprovenance characterized by quartzose sand with low P/F ratio, and Uplifted Basement subprovenance characterized by quartzo-feldspathic sand. The occurrence of more lithic sand wherever erosion fails to completely remove supracrustal

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sedimentary or metamorphic rocks overlying crystalline basement was acknowledged, and Transitional Continental subprovenance formalized later on (Dickinson et al., 1983). These three subdivisions, broadly equivalent to Krynine's and confirmed by studies of modern rift settings, define an unroofing trend (Garzanti et al., 2001). To emphasize this aspect, the corresponding labels "Undissected", "Transitional" and "Dissected" can be used also for Continental Block provenance, and not for Magmatic Arc provenance only. Not only arc crust (Dickinson and Suczek, 1979) or continental crust (Garzanti et al., 2006), but even ophiolitic allochthons including oceanic crust and underlying mantle can be envisaged as a progressively eroding multilayer source of sediment (Ophiolite provenance; Garzanti et al., 2002b). The expected petrofacies successions associated with unroofing of deeper tectono-stratigraphic levels in these three distinct cases define the building blocks of the dynamic provenance model presented here in its essence. How to deal with the complexities caused by mixing of sediments with different provenances in anorogenic and orogenic settings will be briefly discussed next.

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5.1. Unroofing of arc crust

As envisaged in the simplest way, arc crust consists of andesitic volcanic covers made of vitric

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groundmass, plagioclase and pyroxenes, overlying tonalitic/granodioritic batholiths made of quartz,

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plagioclase, K-feldspar, biotite and hornblende. Ideal unroofing trends are thus characterized by a progressive increase in quartz, K-feldspar and hornblende at the expense of volcanic lithics and

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pyroxenes, with consequent transition from feldspatho-lithic volcaniclastic sand rich in pyroxenes (Undissected Magmatic Arc provenance) to quartzo-feldspathic plutoniclastic sand rich in amphibole (Dissected Magmatic Arc provenance; Marsaglia and Ingersoll, 1992; Garzanti and

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Andò, 2007b).

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5.2. Unroofing of continental crust

Continental crust has a heterogeneous polymetamorhpic structure inherited from a series of

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previous orogenic cycles. It can be idealized as a tectono-stratigraphic multilayer composed, from

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top to bottom, of unmetamorphosed cover strata, upper-crustal anchimetamorphic to greenschistfacies metasediments, middle-crustal amphibolite-facies gneisses and granitoids, and lower-crustal

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granulites and mafic intrusions. Metamorphic grade, rock density, abundance of plutonic protoliths, and ratio of mafic to felsic products all tend to increase from shallower to deeper tectonostratigraphic levels (Handy 1990).

Sands derived from continental blocks have been classically thought of as consisting chiefly of

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quartz and feldspar, and thus fundamentally distinguished according to their quartz/feldspar ratio, that increases with weathering or recycling and decreases with active tectonic uplift (Folk, 1980 p.130-133; Dickinson, 1985). Sands released from supracrustal rocks, however, may range from quartzose to lithic sedimentaclastic (Fig. 2), including a variety of rock-fragment types and very poor heavy-mineral suites dominated by zircon, tourmaline and rutile (Undissected Continental Block provenance). Mid-crustal crystalline basements shed instead feldspatho-quartzose to quartzofeldspathic sand with rich hornblende-dominated suites (Dissected Continental Block provenance). The Metamorphic Index MI increases steadily when and where erosion cuts deeper into the crustal multilayer (Garzanti et al., 2006). 5.3. Unroofing of oceanic lithosphere Tectonically accreted or obducted sections of oceanic lithosphere that escaped subduction and orogenic metamorphism consist of thin abyssal siliceous oozes mantling pillow basalts made of

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vitric groundmass, plagioclase laths and pyroxene, overlying in turn diabase dykes yielding epidote and actinolite grown during oceanic metamorphism. The underlying gabbros or gabbro-norites, with small plagiogranite bodies at the top and ultramafic cumulates at the base, are mostly made of calcic

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plagioclase, pyroxenes, hornblende and olivine. Commonly harzburgitic mantle peridotites

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represent the base of the ophiolitic multilayer. During unroofing, feldspatho-lithic basalticlastic sand rich in lathwork volcanic grains and pyroxene (Undissected Ophiolite provenance) is ideally

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replaced by lithic ultramaficlastic sand dominated by lizardite-serpentinite rock fragments and rich in enstatite, olivine and minor Cr-spinel (Dissected Ophiolite provenance; Garzanti et al., 2002b). Orthopyroxene-phyric

boninite grains may be common in detritus from

undissected

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5.4. Mixed provenance in anorogenic settings

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suprasubduction-zone ophiolites (Fig. 3; Garzanti et al. 2000).

Lithologic assemblages exposed at divergent plate margins include continental basements and cover

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rocks stripped down to various levels along the rift shoulders (Continental Block provenance), as well as rift-related volcanic rocks shedding feldspatho-lithic volcaniclastic sand rich in

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clinopyroxene (Anorogenic Volcanic provenance; Fig. 2). Where pull-apart basins are tectonically inverted during late rifting stages, recycling of syn-rift clastic successions may produce quartz-rich

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sands with poor heavy-mineral suites (Recycled Clastic provenance; Garzanti et al., 2014). Sediments generated in anorogenic settings, such as those found in diverse tracts of the huge Nile

al., 2015b).

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catchment, can thus be modeled as mixtures of these three end-member provenances (Garzanti et

5.5. Mixed provenance in orogenic settings Modeling orogenic detritus poses difficult problems, solved in the Dickinson model by the loose definition of three orogenic settings and corresponding subprovenances (subduction complex, collision orogen/suture belt, foreland uplift/fold-thrust belt) lumped under the generic label of Recycled Orogen provenance (Dickinson and Suczek, 1979). Because of unclear operational criteria, such subcategories were seldom used subsequently. A more articulate solution is based on two successive logical steps (Garzanti et al., 2007). First, a limited number of orogen archetypes are identified (Himalayan-type collision orogens, Andean-type cordilleras, Oman-type obduction orogens, Apennine-type thin-skinned belts, Indo-Burman-type subduction complexes). These are not just variants within a single orogenic spectrum, but represent different geological objects generated by subduction processes with different geometry and involving plates of different nature

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(continental or oceanic) in the footwall or hangingwall. Consequently, they are made of different materials and shed sediments with different composition. Second, orogens are seen as resulting from the juxtaposition and superposition of a limited number of tectonic domains arranged in

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subparallel linear belts that jointly contribute mixed detritus to the associated sedimentary basins

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(Dickinson and Suczek 1979 p.2176). Five types of such elongated geological domains can be identified as the primary building blocks of all composite orogens (magmatic arcs, obducted or

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accreted ophiolites, neometamorphic axial belts, accreted remnants of rifted continental margins, accreted clastic wedges), each producing predictable detrital modes, heavy-mineral assemblages and unroofing trends (Fig. 3). Such five primary provenances (Magmatic Arc, Ophiolite, Axial Belt,

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Continental Block, Recycled Clastic) can be recombined to describe the full complexities of mixed

5.6. The upgraded provenance model

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detrital signatures produced by erosion of different types of orogenic prisms.

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The provenance model presented here considers several essential aspects neglected in the Dickinson model (i.e., rift-related settings, anorogenic volcanism, ophiolitic allochthons, recycling), but

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includes only six (three more) primary provenance types (Continental Block, Magmatic Arc, Ophiolite, Axial Belt, Anorogenic Volcanic, Recycled Clastic). Continental Block and Recycled

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Clastic provenances are common to both anorogenic and orogenic settings, because detrital modes cannot distinguish between authochtonous and allochthonous continental crust, and because recycling may occur along both divergent and convergent plate margins. Recycled Orogen

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provenance is dropped in favor of Axial Belt provenance, which has a more restricted definition (Garzanti et al., 2010b).

The dynamic upgraded model allows for progressive changes of sediment mineralogy in space and time during the erosional evolution of the same source area. The trends of unroofing defined for different primary provenances show wide overlap on the QFL diagram (Fig. 3), and several converge towards quartzo-feldspathic and hornblende-rich "ideal arkose" resulting from erosion of mid-crustal granitoid rocks similarly exposed in dissected continental blocks, dissected arc massifs or dissected orogenic belts (Dickinson, 1985; Carrapa and Di Giulio, 2001). The QFL plot thus cannot be used to infer provenance directly, but remains valid to outline mixing of detritus from diverse end-member sources (Dickinson, 1988), or as a basic graphic tool for classification (Fig. 1). The most robust key to provenance diagnosis lies in the rich mineralogical and textural spectrum displayed by rock fragments, as in the equally varied heavy-mineral suites where not depleted drastically by diagenetic dissolution.

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6. Concluding remark Detrital modes of sandstones cannot tell us the plate-tectonic setting in which they were produced, because petrography alone cannot discriminate allochthonous versus authochthonous, orogenic

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versus anorogenic, or young versus old parent rocks. If we pay detailed attention to rock fragment

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types and to both relative and absolute heavy-mineral concentrations, then sandstone composition

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can reveal the tectono-stratigraphic level reached by erosion in continental, arc and oceanic sourcerock domains. If we set ourselves free from obsolete or simplistic views and refrain from mythic thinking (Dickinson, 2003), then plate-tectonic processes and their evolution in space and time can

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be eventually unveiled by using an integrated set of bulk-sediment, multi-mineral and singlemineral techniques, coupled with the indispensable complementary geological information.

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ACKNOWLEDGMENTS

Hilmar von Eynatten and Istvan Dunkl kindly invited me to present these thoughts in the form of a

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talk at the 2°WSGS held in Goettingen in June 2014. The ideas exposed herein stem from fruitful

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continuing collaboration with many friends around the world including Yani Najman, Pieter Vermeesch and Xiumian Hu, and enjoyable strenuous work carried out through the years with

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Sergio Andò, Giovanni Vezzoli and our beloved undergraduate and postgraduate students at Milano-Bicocca. Careful acute reviews by Abhijit Basu and Gert Weltje, and precious comments by

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Hilmar von Eynatten and Alberto Resentini, are gratefully acknowledged.

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Garzanti, E., Andò, S., Vezzoli, G., Dell‟Era, D., 2003. From rifted margins to foreland basins: investigating provenance and sediment dispersal across desert Arabia (Oman, UAE). Journal of Sedimentary Research 73, 572-588.

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FIGURE CAPTIONS

Figure 1. Simple descriptive petrographic classification for sands and sandstones. Following the

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nomenclature scheme introduced by Crook (1960) and Dickinson (1970), sands and sandstones are

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classified according to their main components exceeding 10% QFL. An adjective reflecting the dominant rock-fragment type may be added freely (e.g., volcaniclastic, sedimentaclastic,

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metamorphiclastic, ultramaficlastic; Ingersoll 1983). The lithic pole L should include carbonate and chert grains. Shown to the upper right is the same diagram in logratio space (Weltje, 2006).

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Figure 2. Composition of modern sands generated along divergent plate margins (after Garzanti et al., 2001, 2013a, 2013c, 2014, 2015 plus unpublished data; the lithic pole L includes carbonate and

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chert grains). These four datasets, covering passive margins sands around southern Africa, much of the East Africa and Red Sea-Levant rift system from Botswana to Syria and the entire Nile catchment, demonstrate that detrital modes cannot be used as a proxy for plate-tectonic setting. A)

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Anorogenic and orogenic sands overlap extensively, and cannot be discriminated by their QFL

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modes. B) Sands of Dissected Continental Block provenance are controlled strictly by climaterelated weathering. Sands of Undissected to Transitional Continental Block provenance may plot in

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the Recycled Orogen field. The Anorogenic Volcanic and Magmatic Arc fields overlap completely. C) Quartzo-feldspatho-lithic suspended load derived from Ethiopian volcanic highlands mixes with feldspatho-quartzose bedload shed by crystalline basement exposed in the Blue Nile catchment; quartzose White Nile sands are coarser than volcaniclastic Atbara sediments. Main Nile sediments

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thus overlap all three Dickinson's (1985) provenance subdivisions, depending on their grain size: silty levees plot in the Magmatic Arc field (MA), fine bar sands in the Recycled Orogen field (RO), and medium bar sands in the Continental Block field (CB). D) Sediments eroded from rift shoulders are mixtures in virtually all proportions of detritus from crystalline basement (Dissected Continental Block provenance), sedimentary covers (Undissected Continental Block provenance) and volcanic fields (Anorogenic Volcanic provenance).

Figure 3. Composition of modern sands generated along convergent plate margins (after Garzanti et al., 2007; the lithic pole L includes carbonate and chert grains). The five primary types of orogenic provenance overlap widely on the QFL plot, which forbids its uncritical use as an oracle. The key information lies in the rock fragments. A quick glance at the microscope is sufficient to discriminate volcaniclastic to plutoniclastic sands of Magmatic Arc provenance, basalticlastic to ultramaficlastic sands of Ophiolite provenance, metamorphiclastic sands of Axial Belt provenance, sedimentaclastic

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to basementaclastic sands of Continental Block provenance, and lithic to quartzose sands of Recycled Clastic provenance. Grey arrows highlight unroofing trends for each provenance. Fields after Dickinson (1985): MA= Magmatic Arc; CB= Continental Block; RO= Recycled Orogen.

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Coloured scale bars are all 250 m.

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