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Chapter 2. Composition of the continental crust. I. Introduction – major interest?. Interest?. study of the continental crust is critical to our understanding of the origin & differentiation of the Earth. 1. Total mass = 0.6% silicate earth
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Chapter 2. Composition of the continental crust I. Introduction – major interest?
Interest? study of the continental crust is critical to our understanding of the origin & differentiation of the Earth. 1. Total mass = 0.6% silicate earth 2. Main reservoir for K-U-Th (heat) and mineral resources 3. Primary archive of the earth history
II. Upper continental crust (UCC) Most accessible; but also most complicated and differentiated. About 30% of the continental area is submerged beneath the oceans. (a) Precambrian shields and platforms (cratons) - structure well-known, with Z = 35 - 45 km; Vp = 5.8 - 6.4 km/sec (UCC), 6.5 - 7.2 km/sec (LCC) (b) Conrad discontinuity - present or absent (c) Orogenic belts - crustal structures very complicated. In some areas, the Moho is transitional, rather than discontinuous. Methods for determining the composition of the UCC (a) Using geological maps to obtain weighted averages (Clarke, 1889; Clarke and Washington, 1924). (b) Analysis of composite samples of large surface areas (Shaw et al., 1967). (c) Geochemical approach - analysis of fine-grained sediments (shales or loess) and determine the composition of insoluble elements. Estimation of other elemental abundances from a variety of geochemical principles (Goldschmidt, 1933; Taylor and McLennan, 1985; Rudnick and Fountain, 1995).
Taylor-McLennan (1985) approach - to determine the UCC composition Assumption - taking fine-grained sediments to represent the average UCC. Method -analysis of a group of elements which are resistant to fractionation by the processes of sedimentary differentiation (weathering, erosion, transport and diagenesis). From these data, they further applied the geochemical principles to determine the concentrations of other elements, hence established the bulk composition of the UCC. Factors to consider: effects of alteration, erosion, solubility of minerals, residence time (), transport physics, deposition, diagenesis and metamorphism. (a) Rate of physical erosion ≈ 4 x rate of chemical erosion (p. 18; Table 2.4) Variation in rate of erosion: chemical ≈ 3 to 4; physical ≈ 25 (strongly related to tectonic activity) (b) Effects of sedimentary differentiation (mainly by granulometric separation) (c) Effects of diagenesis: Erosion and transport - generally in oxidizing conditions. Diagenesis - in less oxidizing or reducing conditions, it may change the geochemcial behavior (ex., solubility) of some elements.
Effect of sedimentary differentiation Refractory element ratios (La/Yb; Eu/Eu*, La/Th, La/Sc) unchanged (Table 2.5). Effect of heavy minerals on REE: zircon is very important! (Zr) zircon = 50% (Zr) shales = 200 ± 100 ppm (Zr) UCC ≈ 190 ppm.
Non-fractionated elements (2)Elements not fractionated by sedimentary processes are used to determine the UCC composition: What are they, non-fractionated elements? In general, elements having small KSW (<10-6) and short (< 103 yrs): Ti group (Ti, Zr, Hf) - good, but they are controlled by heavy minerals. Al group (Al, Ga) - OK, but they do not vary much in igneous rocks. Fe, Mn - maybe good, but they change solubilities with oxygen fugacity. REE (+ Y, Sc), Th, Nb - the best (3) REE (shale) ≈ 20% higher than REE (UCC) (p. 32) Why? 3 observations: (a) [La]sh ≈ 20% higher than [La] of "shield composite". (b) REE (shale) ≈ 10-30% higher than REE (greywacke or coarse sediments) (c) Clastic sediments = 70-80% total sediments (other 20-30% = carbonates + evaporites) but represent about 100% of REE budget.
Residence time vs. partition coefficient (Ksw) Ti group (Ti, Zr, Hf) - good, but they are controlled by heavy minerals. Al group (Al, Ga) - OK, but they do not vary much in igneous rocks. Fe, Mn - maybe good, but they change solubilities with oxygen fugacity. REE (+ Y, Sc), Th, Nb - the best
C. Determination of the UCC composition Starting from REE and Th 1. REE: fine-grained clastic sediments x 0.8 2. Th: from sediments La/Th = 2.8 => La = 30 ppm From the established geochemical “rules”, Th/U = 3,8 => U K/U = 10000 => K K/Rb = 250 => Rb Rb/Cs = 30 => Cs Nb/Th = 2.3 => Nb Nb/Ta = 11 => Ta Zr/Nb = 7.6 => Zr Zr/Hf = 33 => Hf , .........etc.
REE patterns of shales and loess Characteristics: patterns extremely uniform, almost regardless of their provenances
D. Conclusion Bulk composition of UCC ≈ granodiorite Thickness ≈ 10 km (≈ 25% of CC; constrained by heat flow data).
III. Lower continental crust (LCC) References: Rudnick and Gao, 2004; Rudnick and Fountain, 1995, Rev. Geophys., 33: 267-309; Taylor and McLennan, chapter 4, 73-95; Ringwood, 1975) Definition: Taylor-McLennan (1985): lower crust (LC) = below 10 km. Rudnick-Fountain (1995): middle crust (MC) = 10-25 km. lower crust (LC) = below 25 km. Badly known in general. LC is probably composed of granulite facies rocks and MC amphibolite facies rocks. Granulites and amphibolites often occur in Precambrian terranes or as xenoliths in alkaline basalts. At present, the composition of LC is still the principal source of uncertainty in the estimation of the bulk composition of the continental crust for the following reasons: large difference in composition between granulites of Precambrian terranes (mainly felsic rocks) and xenoliths (mainly mafic rocks). LC is highly heterogeneoous in lithology, as witnessed in granulite terranes. difficult to determine the types of rocks from seismic data (Vp-Vs).
Seismic velocities in the crust Vp and Vs are related to some physical parameters (T, P, porosity, fluid, etc.) and intrinsic properties of rocks (mineralogy, chemical composition, metamorphic grade, oreintation of grains, etc.) Fig. 2: Variation de Vp pour 8 sections types de la CC (Rudnick et Fountain, 1995). Precambrian shields and platforms Paleozoic orogenic belts (Hercynian, Caledonian, Appalachian) Mesozoic-Cenozoic extensional regions (Basin and Range) Continental arcs (Cascades; Honshu, Japan) Forearcs (Coast of British Columbia; S. California; NE Japan) Rifted margins (W. Norway; Bay of Biscay; eastern USA; W. Greenland) Mesozoic-Cenozoic orogenic belts (Alps, Himalayas, Canadian Cordillera) Active rifts (Rheingraben, Kenyan rift, Rio Grande rift, Dead Sea)
B. Granulite Vp’s Granulites - Considered as the dominant rock-type in LCC. They occur in old granulite terrains or as xenoliths in alkaline basaltic magmas. Fig. 3: Contrast in composition between two types of granulites. Isobaric cooling = products of "magmatic underplating" Isothermal uplifting = supracrustal sequences (or “greenstone-granite assemblages”). Xenoliths = mafic lithology predominant. Interpretation: Xenolithes of mafic granulites probably represent underplated basalt magma crystallized directly into the granulite facies. Critique: LCC xenoliths occur uniquely in extensional regions where alkaline basalts are erupted. Therefore, the lithology of the xenoliths may not be representative of the bulk of LCC.
Xenoliths Isobaric cooling = products of "magmatic underplating” Isothermal uplifting = supracrustal sequences
Isobaric cooling Isothermal uplifting or exhumation Isobaric cooling = products of "magmatic underplating” Isothermal uplifting = supracrustal sequences
IV. Rudnick-Fountain method (1995) Assign the types of rocks that match observed Vp data. Use the geochemical data of granulites compiled by Rudnick and Presper (1990) to obtain the average composition of each layer and of each crustal section of Fig. 2. Estimate the surface area of each crustal section (Fig. 2). Calculate individual volumes using the crustal thickness of each section. Calculate the mean composition of LCC, MCC and the bulk CC. Fig. 5 - Vp vs density diagrams Fig. 7 - Vp vs lithology diagram Mean Vp of the LCC in all sections = 6.9 - 7.2 km/sec. => corresponding to basic granulite, amphibolite, anothosite, and high-grade metapelites. Very rare rock types: basic eclogites and ultrabasic rocks.
Chosen models for LCC (1) High Vp layer (≥ 6.9 km/sec): 6.9 km/sec: 40% basic granulite, 25% interm. granulite, 25% acid granulite, et 10% metapelite. 7.2 km/sec: 90% basic granulite, 10% metapelite. (2) Intermediate Vp layer (6.5 - 6.9 km/sec): 45% interm. gneiss, 45% mixture of amphibolite + felsic gneiss, 10% metapelite. (3) Low Vp layer (6.2 - 6.5 km/sec): 50% interm. granulite, 50% felsic granulite. Result: Tables 9 & 10: Composition of LCC Conclusions: (1) Composition of LCC = basic, ≈ primitive basalt (Fig. 11) (2) Composition of MCC = intermediate (Fig. 11). (3) Composition of the bulk CC = intermediate. (4) Granulite terrains = represent MCC (P = 6 - 8 kb) (5) Xenoliths = root part of the CC (P = 10 - 15 kb) Fig. 12: Trace element patterns of the model CC. Fig. 13: Proportion of lithophile elements in the CC.
V. Model for the CC bulk composition The bulk composition of the CC depends on the choice of LCC models and mechanism for the generation of CC. Approches: (1) estimate the composition of LCC and calculate the bulk composition of CC. (= Approch of Rudnick et Fountain) (2) choose a crustal growth model and identify its products. (= Approch of Taylor et McLennan, 1985, Chap. 3, 57-72). a) lateral accretion of island arcs b) crustal "underplating" (tonalitic or dioritic intrusion) = a variant of the "andésite” model. Andesite Model:Continental growth via addition of andesites and associated rocks in orogenic zones. This is followed by intracrustal melting, leading to the formation of UCC and LCC. (Model valid for post-Archean times) Model of Archean bimodel magmatism: Component 1: Basic = basalts, komatiites Component 2: Felsic = TTG., acid volcanic rocks Andesite ≈ 2:1 mélange of Comp 1 + Comp 2
Bulk composition of the total crust Constraints: Must satisfy the heat flow data. Must have a composition capable of generation of granodiorites by partial melting. Contribution of island arc volcanisms (post-Archean): limited to 25%. Principal period of crustal growth (75%) in the Archean. Total crust = 75% Archean crust + 25% "andesite" SiO2 = 57.3% Cr = 185 ppm, Ni = 105 ppm K = 0.91%, U = 0.91 ppm Th = 3.5 ppm Sm/Nd = 0.22 Rb/Sr = 0.12 (La/Yb)N = 4.9 Eu/Eu* = 1.0 K/U = 10000 Th/U = 3.8
