Geochemical discrimination of elastic sedimentary ...

ELSEVIER

Sedimentary

Geology

113 (1997) 111-124

Geochemical discrimination of elastic sedimentary rock sources P.W. Fralick *, B.I. Kronberg Depament

of Geology, Lukehead Vniversi@ Thunder Bay, ON P7B 5E1, Canada Received

28 February

1996; accepted

13 March 1997

Abstract Factors controlling the geochemistry of a elastic sedimentary rock can include: (1) composition of source terrain, (2) chemical weathering, (3) hydraulic sorting, (4) diagenesis, (5) metamorphism, and (6) hydrothermal alteration. A linear solution inferring source terrain composition from geochemistry of the sediment is impossible in this multivariate system as several unknowns will commonly be present. The use of graphical analysis of element pairs circumvents the problem. Chemically immobile elements will maintain invariant ratios during rock mass change caused by either addition or depletion of mobile elements. This results in chemically immobile element scattergrams exhibiting linear trends along radians extending from the origin, if the major mineral phases of the immobile elements have behaved in a hydrodynamically similar manner. As chemically mobile element plots will produce a scatter of points, this relationship can be used to test chemical immobility and similarity in hydrodynamic sorting history. The constraint on analysis is that the source area must be compositionally uniform or the sediment well mixed prior to delivery to the basin. A second technique, using Si02 plots, has also been developed to investigate element mobility and hydrodynamic behaviour of the mineral phases containing the elements. SiOp-immobile element plots result in a linear arrangement of points extending towards either 0% or 100% SiOz. The 0 intercept position occurs for elements with major mineral phases concentrated in sand; the 100 intercept for those concentrated in clays. Plotting chemically mobile elements produces different patterns, and this can be used to gain information on alteration and sorting history. Elemental ratios for chemically immobile elements with similar hydrodynamic behaviour will be the same as those for the weighted average composition of the source material. This provides a powerful tool for deducing source terrain from sediment geochemistry. Techniques outlined above were tested on Archaean metasandstones from Superior Province, Canada. Immobile element ratio diagrams for Nb-Al-Ti and Zr-ALTi indicate that a calc-alkaline extrusive-intrusive suite lying to the north of the study area was the source, and not five other volcanic suites in the region. This conclusion agrees with previous clast lithology studies and accentuates the applicability of the geochemical techniques. Keywords:

elastic sediments; provenance; trace elements; geochemistry; Archaean; Superior Province

1. Introdllction Ascertaining provenance of sand-sized material in ancient sedimentary systems is generally diffi* Corresponding author. Fax: +l 807 343-8023; E-mail: pfralick @flash.lakeheadu.ca

0037-0738/97/$17.00 0 1997 Elsevier Science B.V. Ail rights reserved. PZZ SOO37-0738(97)00049-3

cult due to chemical and physical modification of source materials during weathering, erosion, transport and deposition (Johnsson, 1993) (Table 1). Further compositional changes may occur prior to, as well as during, sediment burial and removal from surficial environments, resulting from the isolated or combined effects of lithification, diagenesis, meta-

112 Table 1 Selected references

P.U? Fralick, B.I. Kmnberg/Sedimentary

on factors controlling

Geology 113 (1997) 111-124

sediment geochemistry

Weathering Webbcr and Jellema, 1965; Kronberg et al., 1979, 1982, 1986, 1987a,b; Nesbitt, 1979; Kronberg and Nesbitt, 1981; Sastri and Sastry, 1982; Kronbcrg, 1985; Stallard, 1985; Kronberg and Melfi, 1987; Lyew-Ayee et al., 1988; Heins, 1993; Palomares and Arribas, 1993; Melfi et al., 1983; Johnsson et al., 1993; Devaney and Ingersoll, 1993; Linn and DePaolo, 1993. Diagenesis Bonatti et al., 1971; Klinkhammer et al., 1982; Sawlan and Murray, Milodowski and Zalawiewicz, 1991; Kronberg and Fralick, 1992. Metamorphism Mueller, 1967; Ayers and Watson,

1991; Van Baalen,

General Condie and Snansieng, 1971; Wybom 1993; Floyd et al., 1991.

and Chappel,

1983; Colley et al., 1984, Jarvis and Higgs,

1987; Jarvis et al., 1988;

1993; Rubin et al., 1993; Stephens et al., 1979.

1983; Hiscott,

morphism and hydrothermal alteration (Table 1). As sediments evolve they exchange energy and materials with their surroundings, and, hence, are open systems and not amenable to linear analysis. On the other hand, nonlinear analyses would be difficult or impossible, due to lack of quantitative information on the physicochemical evolution of source materials. Here, we explore an empirical approach to provenance analysis using immobile minor and trace elements and, in a case study, apply the techniques to determine the provenance of Archaean sedimentary sequences in the Superior Province of Canada. 2. Approach The major problem in using sediment geochemistry to infer source area composition is establishing which elements are immobile. MacLean (1990; see also MacLean and Kranidiotis, 1987; MacLean and Hoy, 1991; Barrett and MacLean, 1991; Barrett et al., 1991) has developed a method for determining elemental mobility during alteration of volcanic rocks, including continuously fractionated series, and suggests that sedimentary rocks with primary continuity of chemical composition are amenable to the procedure. In addition, some of the underlying principles of the techniques put forward here have been discussed by Gresens (1967), Grant (1986) and Argue (1994). The basic tenent of the approach is that immobile elements will increase or decrease in concentration as mobile elements are lost from, or

1984; Sawyer,

1986; Argast and Donnelly,

1987; Girty et al., 1988,

gained by, the rock. The methods are demonstrated in Figs. 1 and 2. In Fig. 1A concentrations of immobile elements A and B, concentrated in clay fractions, are plotted against each other. For this situation, coordinates representing starting composition ‘a’ would move along a tie line intersecting the origin as the system gains or loses mobile elements. If sediment with composition ‘b’ were, subsequently, hydraulically sorted into sand (sst) and clay (sh) fractions, coordinates representing concentrations of daughter clay fractions would plot further up the tie line and coordinates for corresponding sand fractions further down. As total amounts of immobile elements, A and B, are invariant new coordinates for specific degrees of sorting would be proportionately spaced about ‘b’. For the situation in which both immobile elements, A and B, are concentrated in sand fractions, corresponding ‘sst’ and ‘sh’ positions would be reversed with respect to those depicted in Fig. 1A. If elements A and B were to become mobile during sediment evolution, the field of coordinates representing sediment compositions would be other than on a line including the origin, e.g., dotted line (Fig. 1A). For a mobile-immobile element pair (Fig. lB), loss of a chemically mobile element from the system could result in coordinate ‘a’ moving to coordinate ‘b’. Subsequent hydraulic sorting of sediment of composition ‘b’ would yield ‘sst’ and ‘sh’ and result in a linear array of coordinates including the origin. It is assumed that major mineral phases host-

RR! Fralick, B.I. Knmberg/Sedimentary Geology 113 (1997) 111-124 ELEMENTS

CONCENTRATE

113

IN CLAY

a

0-

A

sh

n

B Immobile

B Mobile -

-

A CONCENTRATES

IN SAND,

B CONCENTRATES

:?I

0

/t

B immobile -

IN CLAY

(F)

B Immobile

-W

o

B Mobile -

Fig. 1. Theoretical scattergrams for assessing element mobility. (A) For immobile-immobile pairs preferentially sited in the clay fraction the starting composition ‘a’ moves to ‘b’ during weathering as total sediment mass diminishes. Composition ‘b’ splits into ‘sh’ (clay) and ‘sst’ (sand) due to hydraulic fractionation and forms a line extending to the origin. If the elements remain immobile during postdepositional alteration processes the coordinates ‘sh’ and ‘sst’ will move along the ‘a-b’ vector but will not move off it. If the elements were mobile during a phase of alteration, they will move off the ‘a-b’ vector (one possible path shown by dotted lines in A and B). Plotting of immobile pairs from a number of samples will result in a linear array of points along a line extending through the origin. If the immobile elements both concentrate in the sand fraction, (A) will apply but the ‘sst’ and ‘sh’ coordinates will be. reversed. Scenarios involving other combinations are shown in (B)-(F). Plotting of multiple samples for any of these scenarios results in a scatter of points.

ing elements A and B have similar hydrodynamic behaviour. Other typical scenarios for changes in concentrations of sediments during their evolution are depicted in Fig. lC,D,E,F. Linear arrays of coordinates, including the origin, will result when graphing element concentrations for sample suites with common source terrains, if elements chosen are chemically immobile and major host phases share similar hydrodynamic behaviour; otherwise, data points will be scattered. However, an anomalous linear coordinate array could occur if element pairs share chemical properties, e.g., RE elements. Hence, the conditions for the successful application of this method include: (1) sediments eroded from different areas of a designated source terrain either have similar compositions or were premixed before entering the basin; (2) only elements having

sufficiently different chemical properties should be plotted against each other. Additional information can be obtained on the alteration history of a data set by employing a second technique. This technique rests on the premise that chemical and physical weathering act, over time, to destroy all major mineral phases except quartz as framework constituents of sand. Fig. 2A shows the coordinate pattern for SiO2 plotted against immobile element, A, concentrated in clay fractions. In this situation sediment of composition ‘a’ undergoing hydraulic sorting and chemical alteration would produce clay and sand fractions represented by coordinates ‘sh’ and ‘sst’, respectively. These coordinates lie along a line of negative slope and y-axis intercept at 100% SiO2. Fig. 2B depicts the opposite situation with im-

114

PM? Fralick, B.I. Knmberg/Sedimentary

(A)

3 lO(

I 1

Geology 113 (1997) III-124

(C)

(B)

1

0”

G

0

100

sh

A immobile, con. in clay +

sh A immobile, con. in sand+

(D)

sst

A immobile, con. in both

lE)

(F) sst

sst

0” .ul

a

\

\

0

+

a sh

/

I sh Aa

sh

A mobile,

A mobile ,

con. in clay +

A mobile,

con. in sand +

con. in both

=P

Fig. 2. Theoretical scattergrams derived by plotting SiOp concentrations against those of selected elements. (A) SiO2 concentration plotted against concentration of immobile element ‘A’ enriched in clay fraction. Starting composition ‘a’ yields two new compositions upon sorting into fine (sh) and coarse (sst) fractions. Changes in ,902 content proceed at the same rate as changes in concentration of the immobile element resulting in a vector extending to 100% SiO2 for an immobile element which concentrates in clay (A), and 0% SiO2 for an immobile element which concentrates in sand (B). Arrows denote movement of points if element ‘A’ becomes mobile during a later phase of alteration. Plotting of numerous data points will result in a linear trend extending to either 100 or 0% SiOs for scenarios (A) and (B), respectively. Other scenarios (C)-(F) will result in a scatter of points when multiple samples are plotted.

mobile element A concentrating in sand fractions. Fig. 2C depicts the coordinate pattern, in which immobile element A is partitioned between sand and clay fractions. Fig. 2D,E,F illustrate coordinate arrays with element A chemically mobile. In the latter examples (Fig. 2C,D,E,F) coordinates for a suite of samples would display scattered patterns. 3. Case study 3.1. Geologic setting The methods discussed above were applied in the provenance analysis of samples of Neoarchaean metasedimentary rocks from the Canadian Shield of Ontario. Fifteen medium-grained sandstone samples were collected from the southernmost Wabigoon Subprovince and northern Quetico Subprovince of western Superior Province (Fig. 3A). Sedimentary rocks in the study area were deposited in a fore-

arc basin-trench system during the Late Archaean (Williams, 1986, 1987, 1990; Barrett and Fralick, 1989; Devaney and Williams, 1989; Percival, 1989; Fralick et al., 1992; Eriksson et al., 1994, 1997) (Fig. 3B). The forearc basin assemblage constitutes the southern margin of the Wabigoon Subprovince, a mainly metavolcanic-granitic succession. The forearc basin has been structurally disrupted forming a pattern of three, near-vertical sediment packages underlain by volcanics, each kilometres thick, the Northern, Central and Southern Metasedimentary Belts (N.M.B., C.M.B., S.M.B., in Fig. 3A). The N.M.B. is composed of conglomerates and sandstones deposited on a braidplain from which sediment was distributed to prograded fan and braid deltas in the C.M.B. (Devaney and Fralick, 1985; Devaney, 1987). The lower portion of the C.M.B. and the S.M.B. are composed of a combined turbidite ramp/fan assemblage (Barrett and Fralick, 1985, 1989) from which sediment was delivered

PW! Fralick, B.I. Kmnberg/Sedimentary

Geology 113 (1997) 111-124

115

LEGEND PROTEROZOIC

Intrusive

Rocks

ARCHEAN

Granitic

Rocks

Metasedimentary

Rocks

Felsic and Intermediate Volcanic Rocks : ; cl

.3

Onaman-Tashota

Mafic and Intermediate Volcanic Rocks sample

location

Terrane

Fig. 3. (A) Regional geology and sample locations in the study area. (B) Depositional environments and tectonic setting during the Neoarchaean evolution of the study area. N.M.B. = Northern Metasedimentary Belt; N.VB. = Northern Volcanic Belt; C.M.B. = Central Metasedimentary Belt; C.VB. = Central Volcanic Belt; S.M.B. = Southern Metasedimentary Belt; S.VB. = Southern Volcanic Belt.

to the Quetico trench, where an unstructured sequence of turbidites accumulated (Fralick et al., 1992) (Fig. 3B). Metamorphic grade increases from greenschist facies in the forearc basin assemblage to amphibolite facies in the Quetico accretionary wedge. The Onaman-Tashota volcanic arc terrain lies to the north of the forearc basin. Based on clast compositions and grain-size trends, the Onaman-Tashota terrain has the appropriate composition

and location to be the source area of the sediment (Devaney, 1987). Metasandstones in the area provide a good test case for evaluating element mobility. The clast composition of conglomerates laterally along the N.M.B. and C.M.B. is reasonably similar (Devaney, 1987), indicating minimal lateral and vertical compositional variation of erosion products emanating from the source area. This is important as similar starting

Ba co Cr CU MO Nb Pb Rb Se Sr V Y Zn Zr

LOI Total

p205

K20

MgG CaO NazO

MnO

Fez03

A1203

SiO2 TiOz

440 21 74 27 1.0 4.4 70 57 300 100 86 9 54 14

69.8 0.43 12.7 5.17 0.09 1.65 2.13 3.40 1.41 0.09 1.93 99.5

1

430 29 110 40 1.7 4.0 87 65 310 150 100 8 41 154

66.6 0.57 14.6 4.31 0.08 1.71 2.30 4.17 1.54 0.12 3.00 99.1

2

Forearc fluvial

260 20 70 13 0.1 4.0 73 31 290 170 70 7 45 100

66.4 0.42 12.7 4.52 0.10 2.18 2.91 5.21 0.68 0.10 5.23 100.5

3 64.7 0.56 15.3 5.60 0.08 2.63 2.37 3.98 1.98 0.15 2.08 99.6 710 29 130 49 2.6 6.3 98 68 360 370 100 12 69 100

4

660 18 150 55 0.1 6.2 80 85 370 350 100 12 73 92

64.4 0.57 15.2 5.78 0.09 3.00 2.79 3.51 1.93 0.15 2.00 99.7

5

Forearc turbidites

560 21 89 24 2.3 5.0 86 58 320 420 70 11 60 70

65.7 0.44 14.0 4.75 0.10 2.03 4.76 4.03 1.62 0.11 1.93 99.6

6

Table 2 Analysis of metasediments from the Beardmore-Geraldton Region

61.5 0.59 17.2 5.87 0.08 3.26 2.30 3.50 2.78 0.16 2.39 99.8 880 26 120 32 0.1 6.5 100 104 400 350 96 11 78 133

I

670 24 220 10 3.0 5.6 100 86 380 410 97 8 60 92

64.3 0.54 15.7 5.44 0.09 3.54 2.44 3.80 2.13 0.15 1.85 100.2

8

660 21 180 35 0.1 5.4 97 99 370 400 100 8 61 104

63.6 0.55 15.7 5.88 0.08 3.23 2.66 3.92 2.30 0.15 1.08 99.3

9

Trench turbid&es

490 20 110 15 0.9 6.1 110 74 370 510 90 11 48 136

64.8 0.56 16.0 5.48 0.08 2.54 2.25 4.94 1.53 0.15 1.70 100.2

10

680 21 200 20 0.7 5.3 100 96 370 410 100 I 61 93

65.5 0.55 15.6 5.47 0.07 2.97 2.41 3.19 2.17 0.15 1.62 100.5

11

520 24 110 33 0.3 6.1 72 48 280 280 80 14 48 88

62.2 0.59 16.6 6.42 0.10 3.74 2.05 4.19 1.87 1.15 2.31 100.4

12

390 42 170 51 0.4 7.5 86 44 350 330 130 14 83 120

64.8 0.68 13.5 7.16 0.12 3.15 3.84 3.37 1.09 0.12 1.39 99.4

13

410 20 190 47 1.7 6.1 loo 60 350 360 100 10 55 116

60.9 0.60 16.5 6.48 0.10 3.73 2.90 4.39 1.32 1.17 2.54 99.8

14

420 23 160 34 2.0 5.0 88 76 320 440 82 8 54 13

67.1 0.5 1 14.4 4.85 0.09 2.69 3.93 3.82 1.50 0.14 1.08 100.3

15

?J zz

l?K? Fralick, B.I. Kmnberg/Sedimentary

compositions for the sedimentary assemblage is essential for the proper functioning of both techniques. 3.2. Analytical methods Medium-grained metasandstone samples were collected from braided fluvial channel deposits in the N.M.B. and C.M.B., and A and B (Bouma, 1962) divisions of turbidites in the S.M.B. and Quetico accretionary prism. Metasandstones from the forearc basin are greenschist facies, dominated by the mineral assemblage quartz, plagioclase and chlorite; those from the Quetico Subprovince (trench) are amphibolite facies with predominantly quartz, plagioclase and amphibole. Major elements and Cr, Rb, Sr, Y, Zr, Nb and Ba were analyzed using standard XRF techniques. Ba, Co, Cr, Cu, MO, Nb, Pb, Rb, Sr, V, Y, Zn and Zr were analyzed using ICP-AES. Where an element was analyzed by both XRF and ICP techniques the data set with the higher precision and accuracy was included in Table 2. This results in all trace element data, except Rb and Y, being generated by ICP analysis. 3.3. Testing element immobility The first method of testing element immobility involves plotting pairs of elements which are suspected of being immobile, e.g., Al, Ti, Zr, Nb and Y. Element pairs involving AlzOs. TiOz and Nb form linear arrays along lines extending through the origin (Fig. 4D,G,H). Y shows extensive scatter when plotted against other elements. Plots of Zr against Ti and Al produce somewhat linear arrays, but these do not trend towards the origin. The distribution of points in Fig. 4 demonstrates that AlzOs, TiOz and Nb were immobile and also hydraulically fractionated in a similar manner. The slight deviations from linearity may be caused by minor differences in source material, or slight differential hydraulic fractionation of the main mineral phases containing Al, Ti and Nb. Y was either chemically mobilized or was physically partitioned during sediment transport differently than the other elements graphed. The patterns produced by Zr when graphed against chemically immobile elements are enigmatic, but show that its behaviour differs from that of Al, Ti and Nb. Plotting selected elements against SiO2 concen-

Geology 113 (1997) 111-124

117

trations for sandstones and slates provides further insight into chemical and hydrodynamic behaviour of their major mineral phases (Fig. 5). As slates are relatively scarce in the study area, fine-grained Quetico sediments to the west of the study area (Sawyer, 1986) were used to establish an end slate composition. The western area shares similarities in maximum age, source area, and transport processes with the Quetico sediments examined in the current study. Metasandstones analyzed by Sawyer (1986) confirm that these are geochemically very similar to those from the current study area. Fig. 5A,B portray typical trends for elements which are concentrated in the sand fraction (Zr and CaO data from Sawyer, 1986; CaO from this study). Trends in Zr data from the present study indicate that Zr is not preferentially concentrated in either the fine or coarse fraction. This implies the presence of both very small and larger Zr-bearing phases in roughly equal proportions. Fig. 5C,D clearly indicate that TiOz and A1203 concentrate in the clay fraction. Nb (not shown) has a similar trend. These patterns contrast with those of Y and Rb (Fig. 5E,F) which are concentrated in the clay fraction, but are also lost from the system. Samples from this study have undergone greater Y loss than Sawyer’s samples (Sawyer, 1986) . 3.4. Provenance Data presented in the preceding section suggest that Al, Ti and Nb were immobile and affected by sorting in a similar manner. Plots of these elements against Si02 also suggest that Al-Ti-Nb were concentrated in the fine fraction. Hence, ratios of Al to Ti to Nb in the metasediments should be the same as the average ratios for these elements in the source terrain, provided these elements have not been mobile in source terrain rocks after the erosion of the sediment occurred. Six chemically distinct volcanic sequences are present in the study area, ranging from intermediate to mafic and including four sequences of tholeiitic affinity (CVB, SVB, NVB, OTT; Fig. 6) and two of talc-alkaline affinity (CVBCA, O’ITCA; Fig. 6) (Tomlinson et al., 1993). Al, Ti and Nb have remained relatively immobile in these rocks (Tomlinson et al., 1996). A plot involving Al, Ti and Nb for metasandstones from the current study shows a tight

118

RW Fralick, B.1. Knmberg/Sedimentary

Geology 113 (1997) 111-124

0

-

Trench Turbidites

t

I

I

I

l-2

I4

I6

I&

A1203

%

0.8

ap

0” ._ l0.6

0.4

Ii0

i0 8

IO

14

12

Zr

I30

oDm

130 ae k

100

70

6

Zr

I I6

I 16

I

14

I2

120

AlzOS

%

pm

G.

8-

33 0.4

05

TiOz

0.6

%

12

I

I

I

I

I3

I4

I5

IS

A1203

%

Fig. 4. Scattergrams showing immobility of Al, Ti, and Nb as well as mobility and/or physical partitioning of Y. Zr behaviour is enigmatic and not readily explicable. Lines on scattergrams are mass gain-mass loss paths extending from the origin. If both elements plotted were chemically immobile and their major mineral phases behaved in a hydrodynamically similar manner the points will plot along a mass gain-mass loss path. Any other situation causes a scatter of points.

f!W: Fralick, B.I. Kronberg/Sedimentary

Geology 113 (1997) 111-124

I

20 %

Zr pm

3

i

This Study + Fluvial Sandstone * 0”

??Foreorc

Turbidites

0 Trench

Turbidites

70 ap

60

z

Sawyer,

50 I

I

I

.5

.7

0” ‘Z

1986

0

Quetico

Turbidites

-

Ouetico

Metopelites

60

#I Averoge

Puettco Metopelite

X Averoge

Ouetico Turbidite

,:

TiZa %

Y pm D.

70

#

F.

70

* 0” z

0” ‘Z 60

--

A

I I4

I

I5

A1203

lk

I

I7

%

I:,

60

I

50

I

I

I00 Rb

150 pm

Fig. 5. Selected elements plotted against SiOz. Point b = average slate composition (Sawyer, 1986); line b-b’ = theoretical trend if element concentrates in clay fraction; line b-b” = theoretical trend if element concentrates in sand fraction (lines b-b’ and b-b” extend to 100% and 0% SiO2, respectively); line a-a’ = trend of data from this study; line b-c = trend of data from Sawyer, 1986. (A) Zr data from Sawyer (b-r) plots close to line b-b” indicating that zirconium in his samples was chemically immobile and concentrated in the sand fraction. Zr data from the present study forms a horizontal linear trend indicating chemical irnmohility and lack of partitioning into a specific size fraction. (B) Data from Sawyer and this study indicate that Ca was concentrated in the sand fraction and chemically mobile (the concentration in the sand fraction is higher than that expected, line b-b”, for sorting alone). (C) Ti data from both Sawyer and this study indicate that it was chemically immobile and concentrated in the fine-grained fraction. (D) The behaviour of Al was very similar to Ti. (E) Y was chemically mobile showing depletion in the sand fraction greater than that expected by hydrodynamic sorting. Samples from this study exhibit greater Y depletion than samples from Sawyer (1986). (F) Rb exhibits a depletion pattern similar to Y, but with little difference between the two sample sets.

clustering of data with two exceptions (Fig. 6). A tight cluster, such as this, is the expected pattern if the testing of all assumptions has led to the proper choice of elements. The presence of TiOz in the numerators of both ratios (TiO2/Al2Os, TiOz/Nb) will cause a positive linear correlation to occur if the immobile element ratios of the sediment are not fixed in space by a constant ratio in the source material. The separation of a fluvial sample (number 2) from the cluster may be caused by its more proximal position

within the basin leading to less thorough mixing of source area detritus. The trench sandstone, sample 13, which plots well away from the group, must have had, in part, a chemically distinct source. The Onaman-Tashota terrain was the probable source of the sediment in the forearc basin as indicated by clast lithologies in conglomerates and coarsening trends (Devaney, 1987; Devaney and Williams, 1989). The talc-alkaline volcanic assemblage of the Onaman-Tashota terrain is the only extrusive-

120

PU? Fralick, B.I. Kronberg/Sedimentary

Geology 113 (1997) 111-124

+ \

100

X

90

Rocks

METASEDIMENTS + . 0

I I

TiO,

I 2

I 3

/ Nb x 1,000

Fig. 6. AlzOs-TiOz-Nb ratio diagram comparing metasandstones with possible volcanic source terrains (stars, geochemistry of volcanic rocks from Tomlinson et al. (1993); other symbols, as in Fig. 5). 07T = Onaman-Tashota terrain; NVB = Northern Volcanic Belt; CVB = Central Volcanic Belt; SVB = Southern Volcanic Belt; CA = talc-alkaline. Data on volcanic rocks represents two typical analyses of each of the six volcanic suites present in the area.

intrusive suite with immobile element ratios similar to those of the metasandstones. This strongly indicates that the Onaman-Tashota talc-alkaline volcanic rocks formed the arc which fed sediment into the Beardmore-Geraldton forearc basin and Quetico trench. The chemical mobility and hydraulic sorting of Zr can be further explored using a ratio diagram incorporating data from this study and Sawyer (1986) (Fig. 7). Sawyer’s metasandstone samples (Sawyer, 1986) consistently plot along a separate portion of the trend relative to his fine-grained samples. This is caused by hydraulic sorting concentrating the main Zr-bearing minerals in the sands, but the major Al and Ti bearing phases in the silts/clays. If Zr were chemically mobile data points would have scattered rather than display a linear trend, confirming that Zr was retained in the same host mineral phases during weathering and diagenesis. As the Al-TiZr ratios of the average source material will lie somewhere along the line, this diagram is useful in

.

50

fluviol foreorc turbidites trench turbidites

\ +* 0

. \ ??

Sawver ,I986 QUETICO METASEDIMENTS x sondstone - slate

t

I

I

30

40

50

0

??

I 60

I 70

TiO, / Zr Fig. 7. AlzOs-TiOz-Zr ratio diagram. Note separation of metasandstones and finer-grained samples in Sawyer’s data (Sawyer, 1986) caused by differences in hydraulic behaviour between major mineral phases containing Zr and those containing AlzOs-Ti02.

constraining the source composition. Again sample 13 is separated from the main group, indicating that it had a different source. A plot of Al-Ti-Zr ratios for the sedimentary and volcanic suites in the area shows that only the Onaman-Tashota talc-alkaline volcanic and intrusive rocks overlap with the linear trend of the metasandstones (Fig. 8). This again identifies the Onaman-Tashota terrain as the source of the sediment. A ratio diagram incorporating Y shows the effects of its partial removal from the system (Fig. 9). Fig. 5E indicates that Sawyer’s samples (Sawyer, 1986) have lost less Y than samples in this study. This is reflected on Fig. 9 by a difference in the position of the two sample groups. The starting composition must lie even further to the right. Again sample 13 (upper right) plots well away from the others, indicating a different source.

I?U? Fralick, B.I. Kronberg/Sedimentary

Tomlinson

Kresz

Zayachivsky 1989 -p0 mafic volcanic rocks LI intermediate volcanic rocks ??intermediate to felsic sub-volcanic intrusive rocks A intermediate to felsic intrusive rocks 0 intermediate to mafic intrusive rocks

et al, 1993

G# mafic

to intermediate volcanic rocks Onoman - Tashota Terrane Northern Volcanic Belt Central Volcanic Belt Southern Volcanic Belt Colt- Alkaline

OTT NVB CVB SVB

160

CA

121

Geology 113 (1997) III-124

and

Tholeiitic and

volcanic intrusive rocks

RAMAFIC

talc -alkaline volcanic and I 20

intrusive

I 40

rocks

I 60

I 120

I loo

I 80

TiO,/

I 140

I 160

I 180

I 200

Zr

Fig. 8. AlzOs-TiOz--Zr ratio diagram comparing metasandstones with possible volcanic source terrains. Data from Kresz and Zayachivsky (1989) is for tholeiitic and talc-alkaline volcanic suites present in the Onaman-Tashota terrain. Sediment compositions occur along the linear trend running through the Onamat-Tashota talc-alkaline data field, indicating that this was the source of the sandstones.

4. Conclusions

5

??13

0” -$4

\

B Y

0” .I3 J

0

4

8

Ii

Y x. 100,000/AI,0, Fig. 9. AlzOs-Ti02-Y ratio diagram. Separation of data sets is attributed to higher Y mobility in samples from this study compared with Sawyer’s study area. See Fig. 7 for symbols.

The lithogeochemical approaches applied in this study for testing chemical immobility of elements in elastic sedimentary rocks provide a mechanism for refining the analysis of this type of data. The effects of weathering, sorting, diagenesis, metamorphism and hydrothermal alteration on the chemistry of the sediments potentially can be assessed for each element. Elements that were immobile and whose major host mineral phases behaved in a hydrodynamically similar manner can be used to construct ratio plots. Clustering of points on immobile element ratio plots reflects the weighted average composition of the source rocks which contributed sediment. If the source rock composition was variable, and the sediment was not well mixed before deposition, points will scatter on the ratio plots. This may be useful in deciphering multiple source terrains and the sediments derived from each. A spatially and temporally nonvarying source or a spatially varying source terrain from which the sediment has been well mixed results in a tight clustering of points on the ratio plots. These ratios provide a

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quantitative tool for identifying the area(s) of erosion. There may be a tendency to use immobile element ratios to construct plots defining tectonic environment, similar to diagrams of Pearce and Cann (1973) for volcanic suites. This trend would be unfortunate as tectonic interpretation should have a much broader data base, incorporating sedimentological, igneous and structural data, plus, in some areas information from other fields (for additional criticisms see: Mack, 1984; Van de Kamp and Leake, 1985; Zuffa, 1987; Girty et al., 1988; Floyd et al., 1991; Haughton et al., 1991). If this broad base of data is not available and sedimentary geochemistry must be relied on, in conjunction with basin analysis techniques, to interpret tectonic setting, more reliable results will be obtained if the immobile element ratios in the sediment data set are compared with actual analyses from possible source terrains rather than worldwide averages reflecting various tectonic settings. Testing of the methods on samples from the Late Archaean Beardmore-Geraldton and Quetico metasedimentary belts emphasizes their usefulness. While these methods may be used for exploring sediment source terrains at any stage in the sedimentary cycle the results documented here show that the methods are robust even in the advanced stages of metamorphism. Immobile element ratio plots precisely define the source region and even indicate that the volcanism feeding sediment to the forearc basin and trench was mainly andesitic and talc-alkaline. The ability to differentiate between volcanic suites forming possible source terrains of other basins offers the opportunity to define more accurately the tectonic setting of other sedimentary sequences. In the study area the confirmation of Onaman-Tashota terrain talc-alkaline volcanism as the sediment source further confirms the tectonic models put forward for the region (Williams, 1986, 1987, 1990; Barrett and Fralick, 1989; Devaney and Williams, 1989; Percival, 1989; Percival and Williams, 1989; Fralick et al., 1992; Eriksson et al., 1994, 1997). Acknowledgements

Grateful appreciation is extended to Wendy Bourke for wordprocessing the manuscript and Sam

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Spivak for drafting the figures. Earlier versions of the manuscript were critically read by T.J. Barrett, J. Ryan, l? de Caritat, K. Crook, R. Hiscott and G. Girty. We appreciate the many suggestions for improving the paper which were put forward by these individuals. The research was supported by the National Science and Engineering Research Council of Canada operating grants to P.W.F. References Argast, A., Donnelly, T.W., 1987. The chemical discrimination of elastic sedimentary components. J. Sediment. Petrol. 57, 813-823. Argue, J.J., 1994. Mass transfer during Barrovian metamorphism of pelites, south-central Connecticut 1: evidence for changes in composition and volume. Am. J. Sci. 294, 989-1057. Ayers, J.C., Watson, E.B., 1991. Solubility of apatite, monazite, zircon and rutile in supercritical aqueous fluids with implications for subduction zone geochemistry. Philos. Trans. R. Sot. London, Ser. A 335, 365-375. Barrett, T.J., Fralick, PW., 1985. Sediment redeposition in Archean iron formation: examples from the BeardmoreGeraldton Greenstone Belt, Ontario. J. Sediment. Petrol. 55, 205-212. Barrett, T.J., Fralick, PW., 1989. Turbidites and iron formations, Beardmore-Geraldton, Ontario: application of a combined ramp/fan model to Archean elastic and chemical sedimentation. Sedimentology 36, 221-234. Barrett, T.J., MacLean, W.H., 1991. Chemical, mass and oxygen isotope changes during extreme hydrothermal alteration of an Archean rhyolite, Noranda, Quebec. Econ. Geol. 86, 406-414. Barrett, T.J., Cattalani, S., Chartrand, F., Jones, P., 1991. Massive sulfide deposits of the Noranda area, Quebec. II. The Aldermac Mine. Can. J. Earth Sci. 28, 1301-1327. Bonatti, E., Fisher, D.E., Joensuu, O., Rydell, H.S., 1971. Postdepositional mobility of some transition elements, phosphorus, uranium and thorium in deep sea sediments. Geochim. Cosmochim. Acta 35, 189-201. Bouma, A.H., 1962. Sedimentology of some Flysch Deposits: A Graphic Approach to Facies Interpretation. Elsevier, Amsterdam, 169 pp. Colley, S., Thomson, J., Wilson, T.R.S., Higgs, N.C., 1984. Post-depositional migration of elements during diagenesis in brown clay and turbidite sequences in the North East Atlantic. Geochim. Cosmochim. Acta 48, 1223- 1235. Condie, K.C., Snansieng, S., 1971. Petrology and geochemistry of the Duzel (Ordovician) and Gazelle (Silurian) Formations, northern California. J. Sediment. Petrol. 41, 741-751. Devaney, J.R., 1987. Sedimentology and Stratigraphy of the Northern and Central Metasedimentary Belts in the Beardmore-Geraldton Area of Northern Ontario. M. Sci. Thesis, Lakehead University, Thunder Bay, Gnt. Devaney, J.R., Fralick, P.W., 1985. Regional sedimentology of the Namewaminikan Group, northern Ontario: Archean flu-

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