Results

This study conducted LA-ICP-MS zircon U–Pb dating for 45 samples (17 metasedimentary rocks and 28 metaigneous rocks) analyzed for whole rock chemistry.

The analytical results are summarized in Table 6.1 and shown with CL images of analysed zircons and concordia diagrams for all samples in Figures 6.1-6.45. All data tables are in Appendix Table S1.

6.1.2.1. Metasedimentary rocks from the Wanni Complex Garnet–biotite gneiss (14111505)

The zircon grains show dominant ovoid shape with polygonal face and minor rounded, stubby, ovoid and elongated shape and the lengths ranging from 0.02 mm to 0.18 mm. 53 concordant data were obtained from 52 cores and 1 rim of analyzed zircons, which also yielded 15 discordant data. The inherited cores have various internal textures, which are bright-CL clear to blurry sector zoning, dark-CL weakly oscillatory, sector, patchy zoning or non-structural zoning, and are overgrown by thin bright-CL sector zoned rims (Fig. 6.1.1). Some zircon grains with bright-CL sector zoning have no evidence of overgrown rims (Fig. 6.1.1). The grains showed Th/U ratios of 0.08–0.26 and ²⁰⁶Pb/²³⁸U ages of 550–520 Ma which are similar ages detected from overgrown rims (524 Ma) with Th/U ratio of 0.22 (Figs. 6.1.1, 6.1.2). However, the inherited cores with overgrown rims exhibit the ²⁰⁶Pb/²³⁸U ages of 1090–690 Ma and various Th/U ratios of 0.02–0.84 (Figs. 6.1.1, 6.1.2).

Garnet–biotite gneiss (14031701A)

Some zircon grains are rounded and elongated, but others are stubby in shape.

These zircons are 0.03–0.14 mm in long. Analyzed zircons gave 16 concordant data from 9 cores and 7 rims, and 8 discordant data. Their internal textures indicated dark-CL weakly oscillatory zoned to unzoned cores and dark-CL weakly zoned rims bounded by thin bright-CL unzoned domains (Fig. 6.2.1). Bright-CL sector zoned zircons are also present (Fig. 6.2.1). The weakly oscillatory zoned cores showed the

Table 6.1. Summary of LA-ICP-MS U-Pb zircon ages.

Detrital zircon age Igneous protolith age

(Xenocristic zircon age) Metamorphic zircon age Grt-Bt gneiss

14111505 E of Kurunegala 14111505 Grt-Bt gneiss 1090–690 550–520

14031701 Near Kegalla 14031701A Grt-Bt gneiss 2440, 760 600–500

98123005 Near Avissawalla 98123005D Grt-Bt gneiss 1540, 1120–690 630–510

98122203 SE of Colombo 98122203A Grt-Spl-Sil-Crd-Bt gneiss 2560, 1660, 1110–670 600–490

Grt-two-Px granulite

14111401 SW of Dambulla 14111401 Grt-two Px granulite ~880 630–500

Two-Px granulite

14031705 Near Avissawalla 14031705E-1 Two Px granulite 1940 620–500

14111204 W of Habarana 14111204B Two Px granulite ~860 620–490

14111404 N of Kurunegala 14111404C Two Px granulite ~1040 630–510

14031702 Near Kegalla 14031702B Two Px granulite ~990 590–510

14111301 SW of Dambulla 14111301A Two Px gneiss ~980 570–480

Charnockite

14111204 W of Habarana 14111204A Charnockite ~980 530–510

14031702 Near Kegalla 14031702C Charnockitic gneiss ~960 530

14031705 Near Avissawalla 14031705E-2 Grt charnockite 1860 640–620

Grt-Bt gneiss

14111606 SW of Kantale 14111606 Grt-Bt gneiss 1130–690 660–500

13040106 SE of Kandy 13040106A Grt-Bt gneiss 2600–1640 620, 570–540, 500

99122803 SE of Madampe 99122803B Grt-Bt gneiss 1980–1500 640–550

99122201 SW of Madampe 99122201B Grt-Bt gneiss 1860–1450 620–530

14111101 W of Polonnaruwa 14111101 Grt gneiss 2080–1630 630–530

14110901 N of Dambulla 14110901A Grt gneiss 1130–710 620–520

99122510 SW of Ratnapura 99122510A Grt gneiss 1840 630–490

94071904 SE of Kalutara 94071904A Spl bg. Grt-Crd-Bt gneiss 3440, 2940–1820 600–520

Grt-Sil-Bt gneiss

99122704 W of Morawaka 99122704G Spl bg. Grt-Sil-Crd gneiss 2040–1600 620–470

Grt-Opx-Bt gneiss

99122101 NE of Kalutara 99122101D1 Grt-Opx-Bt gneiss 2560, 1010–670 610–500

99122101 NE of Kalutara 99122101D2 Grt-Opx-Bt granulite 2500, 2210, 1720, 870–710 640–470

13040106 SE of Kandy 13040106C Grt-Opx-Bt gneiss 1820–1530 630–490

Siliceous gneiss

13040102 SE of Kandy 13040102A Siliceous Grt-Sil gneiss 3050, 2700–1870 640–580

Grt-two-Px granulite

14111607 SW of Kantale 14111607A Grt-two Px granulite ~850 610–510

14111103 NE of Polonnaruwa 14111103B Grt-two Px granulite ~980 620–540

14110802 SE of Dambulla 14110802D Grt-two Px granulite 1810 650–500

13040102 SE of Kandy 13040102D Grt-two Px granulite 630–500

14031606 SW of Nuwara Eliya 14031606B Grt-two Px granulite 640–500

99122807 N of Madampe 99122807 Grt-two Px granulite 1760 (2870, 2640) 650–510

13033102 S of Gampola 13033102A Grt-two Px granulite 1780 640–560

13040106 SE of Kandy 13040106F Grt-two Px granulite 2010 630–530

Two-Px granulite

13040102 SE of Kandy 13040102B Two Px granulite 1850 640–490

13040106 SE of Kandy 13040106G Two Px granulite 1760 650, 580–530

99122510 SW of Ratnapura 99122510L Two Px granulite 620–470

14111610 NE of Kantale 14111610 Two Px granulite 1860 (2570–2010) 630–530

Charnockite

14110904 NW of Polonnaruwa 14110904A Grt charnockitic gneiss ~810 630–510

99122201 SW of Madampe 99122201A1 Grt charnockite 1820 610–510

99010301 NE of Galle 99010301A Grt charnockite 1950 620–490

Felsic Grt-Opx granulite

14110802 SE of Dambulla 14110802B Felsic Grt-Opx-Bt gneiss 1890 640–520

99122704 SE of Madampe 99122704C Felsic Grt-Opx granulite 620–490

Hbl-Bt gneiss

14111604 SW of Kantale 14111604C Grt-Hbl-Bt gneiss 1900 640–520

99122803 SE of Madampe 99122803C Grt bg. Hbl-Bt gneiss 1770 (1890) 580–520

Wanni Complex (Kehelpannala, 1997)

Highland Complex (Kehelpannala, 1997)

Location No. Sample No. Rock name Zircon U-Pb age (Ma)

Location

Fig. 6.1.1.(Continued).

0.04 0.2

data-point error ellipses are 2

207

Pb/

²³⁵

U

206

P b /

238

U

0.6 1.0 1.4 1.8 2.2 2.6

0.08 0.12 0.16 0.20

Metamorphic and resetdomains (gray) 550-520 Ma

1000

400

800

1200

600

14111505 (WC) Grt-Bt gneiss

Inherited domains (green) 1090-690 Ma

10

1

0.1

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h /U

206

Pb/

²³⁸

U age (Ma) Inherited domain

Metamorphic and reset domain

Discordant data

Fig. 6.2.1. Cathodoluminescence images of analyzed zircon grains of garnet‑biotite gneiss (14031701A) in the Wanni Complex. Analyzed spot,²⁰⁶Pb/²³⁸U age and Th/U ratio are also shown.

10

1

0.10

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h /U

206

Pb/

²³⁸

U age (Ma)

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0 2 4 6 8 10 12

600 1000

1400

1800

2200

2600

14031701A (WC) Grt-Bt gneiss

data-point error ellipses are 2

206

P b /

238

U

207

Pb/

²³⁵

U

Inherited domains (green) 2440, 760 Ma

500 600

700 800

900

Metamorphic and resetdomains (gray)

600-500 Ma

Inherited domain

Metamorphic and reset domain

Discordant data

206Pb/²³⁸U ages of 2440 Ma and 760 Ma with Th/U ratios of 0.16 and 0.25, respectively (Figs. 6.2.1, 6.2.2). On the other hand, other domains exhibited younger ages of 600–

500 Ma and lower Th/U ratios of 0.00–0.04 than cores (Figs. 6.2.1, 6.2.2).

Garnet–biotite gneiss (98123005D)

The zircons in this sample have the length of 0.04–0.17 mm and rounded elongate or stubby shapes. In total, 70 concordant data (48 cores and 22 rims) and 22 discordant data were obtained. The analyzed zircons showed cores with various internal textures such as dark- to bright-CL oscillatory, bright-CL sector zoned, dark-CL blurry to unzoned, and overgrown rims with dark-CL weakly zoned or bright-CL sector zoned textures (Fig. 6.3.1). The ages of cores are 1540, 1120–700 Ma and minor 530–520 Ma with various Th/U ratios of 0.01–2.22 (Figs. 6.3.1, 6.3.2). The rims of zircons yielded the ²⁰⁶Pb/²³⁸U ages of minor 700–690 Ma and 630–510 Ma with low Th/U ratios of 0.00–0.19 (Figs. 6.3.1, 6.3.2).

Garnet–spinel–sillimanite–cordierite–biotite gneiss (98122203A)

Zircon grains in sample 98122203A are rounded ovoid and elongate with minor stubby in shape and 0.02–0.2 mm long. 67 concordant data (54 cores and 13 rims) and 23 discordant data were obtained from the zircons. The cores of analyzed zircons are oscillatory, sector or nebulously zoned with darkish- to bright-CL, while the rims are clear to blurry sector zoned with dark- or bright-CL (Fig. 6.4.1). The former provided

206Pb/²³⁸U ages of 2560, 1660, 1110–670 Ma with minor 520–510 Ma and wide range of Th/U ratios of 0.01–1.85, while the latter did ²⁰⁶Pb/²³⁸U ages of 600–490 Ma except one datum (740 Ma) and low Th/U ratios of 0.00–0.14 (Figs. 6.4.1, 6.4.2).

6.1.2.2. Metasedimentary rocks from the Highland Complex Garnet–biotite gneiss (14111606)

The zircon grains exhibited 0.04–0.25 mm in long and, dominant rounded elongate shape. The analysis of these zircons provided 111 available data of 87 concordant from 37 cores and 50 rims, and 24 discordant data. The CL images illustrated their internal

Fig. 6.3.1.(Continued).

Fig. 6.3.2. Concordia diagram and plots of²⁰⁶Pb/²³⁸U age vs. Th/U ratio using available age data obtained from garnet‑biotite gneiss (98123005D) in the Wanni Complex.

0.0 0.1 0.2 0.3 0.4

0 2 4 6 8

600

1000

1400

1800

98123005D (WC) Grt-Bt gneiss

data-point error ellipses are 2

206

P b /

238

U

207

Pb/

²³⁵

U

600

800

1000

1200

Metamorphic and resetdomains (gray)

630-510 Ma

Inherited domains (green) 1540, 1120-690 Ma

10

1

0.10

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h/ U

206

Pb/

²³⁸

U age (Ma)

Inherited domain

Metamorphic and reset domain

Discordant data

Fig. 6.4.1.(Continued).

10

1

0.1

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h /U

206

Pb/

²³⁸

U age (Ma)

Fig. 6.4.2. Concordia diagram and plots of ²⁰⁶Pb/²³⁸U age vs. Th/U ratio using available age data obtained from garnet‑spinel‑sillimanite‑cordierite‑biotite gneiss (98122203A) in the Wanni Complex.

Inherited domain

Metamorphic and reset domain Discordant data

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0 2 4 6 8 10 12 14

600 1000

1400

1800

2200

2600

98122203A (WC) Grt-Spl-Sil-Crd-Bt gneiss

data-point error ellipses are 2

206

P b /

238

U

207

Pb/

²³⁵

U

400

600

800

1000

Metamorphic and resetdomains (gray)

600-490 Ma

Inherited domains (green) 2560, 1660, 1110-670 Ma

textures with core-rim relations (Fig. 6.5.1). Especially, the zircon grains with CL Nos.

66 and 121 consist of core, mantle and rim with clear different zoning patterns and ages (Fig. 6.5.1). The cores have several zonings of dark-CL obscure one, dark- to bright-CL oscillatory one and bright-CL sector one (Fig. 6.5.1). The ages and Th/U ratios obtained from the cores were 1130–690 Ma and 0.30–1.10 with some data of 640–530 Ma and 0.01–0.16, respectively (Figs. 6.5.1, 6.5.2). The rims and rare mantles were dark-CL weakly oscillatory or sector zoned, unzoned, or bright-CL sector zoned in the CL images (Fig. 6.5.1).

Garnet–biotite gneiss (13040106A)

Zircon grains in sample 13040106A are rounded, stubby, elongate and dominantly ovoid in shape, and 0.03–0.13 mm long. These grains exhibit bright-CL clear to blurry oscillatory zoned cores with dark-CL weakly zoned rims (Fig. 6.6.1). 26 concordant data from 20 cores and 6 rims, and 28 discordant data were obtained. 20 inherited cores showed ²⁰⁶Pb/²³⁸U ages of 2600–1640 Ma with high Th/U ratios of 0.32–2.65 (Figs.

6.6.1, 6.6.2). On the other hand, concordant data from dark-CL rims ranged between 620 Ma and 500 Ma with low Th/U ratios (0.01–0.17) (Figs. 6.6.1, 6.6.2).

Garnet–biotite gneiss (99122803B)

The lengths of zircons are between 0.04 and 0.21 mm with the shapes of dominant rounded elongate and less stubby shapes. The dated zircons in this sample put out 46 concordant data from 40 cores and 6 rims, and 30 discordant data. The internal textures of cores are variety as follows: dark-CL clear to blurry oscillatory zoned, dark- to bright-CL sector zoned and dark-CL faint zoned to unzoned textures (Fig. 6.7.1). On the other hand, those of rims are simple as darkish- to bright-CL weakly sector or patchy zoned textures (Fig. 6.7.1). The former produced the ²⁰⁶Pb/²³⁸U ages of 1980–1500, 640–570 Ma and Th/U ratios of 0.10–1.25, while the latter did the ages of 620–550 Ma and Th/U ratios of 0.28–0.84 (Figs. 6.7.1, 6.7.2).

Garnet–biotite gneiss (99122201B)

Zircon grains are 0.05–0.23 mm in long and dominant rounded elongate with less stubby in shape in sample 99122201B. 30 cores and 5 rims of zircons yielded 35

Fig. 6.5.1. Cathodoluminescence images of analyzed zircon grains of garnet‑biotite gneiss (14111606) in the Highalnd Complex. Analyzed spot,²⁰⁶Pb/²³⁸U age and Th/U ratio are also shown.

Fig. 6.5.1.(Continued).

0.0 0.1 0.2 0.3 0.4 0.5

0 2 4 6 8 10 12 14

500 700

900

1100

Inherited domains (green) 1130-690 Ma

14111606 (HC) Grt-Bt gneiss

data-point error ellipses are 2

206

P b /

238

U

207

Pb/

²³⁵

U

600 1000

1400

1800

2200

2600

Metamorphic and resetdomains (gray)

660-500 Ma

10

1

0.10

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h / U

206

Pb/

²³⁸

U age (Ma)

Inherited domain

Metamorphic and reset domain

Discordant data

Fig. 6.6.1. Cathodoluminescence images of analyzed zircon grains of garnet‑biotite gneiss (13040106A) in the Highland Complex. Analyzed spot,²⁰⁶Pb/²³⁸U age and Th/U ratio are also shown.

0 0.0

data-point error ellipses are 2

2 4 6 8 10

0.2 0.4 0.6

207

Pb/

²³⁵

U

206

P b /

238

U

12 14

520 560

600

640 680

480 13040106A (HC)

Grt-Bt gneiss

Inherited domains (green) 2600-1640 Ma

Metamorphic and reset domeains (gray) 620, 570-540, 500 Ma

600 1000

1400

1800

2200

2600

Inherited domain

Metamorphic and reset domain Discordant data

10

1

0.1

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h/ U

206

Pb/

²³⁸

U age (Ma)

Fig. 6.7.1. Cathodoluminescence images of analyzed zircon grains of garnet‑biotite gneiss (99122803B) in the Highland Complex. Analyzed spot,²⁰⁶Pb/²³⁸U age and Th/U ratio are also shown.

Fig. 6.7.2. Concordia diagram and plots of²⁰⁶Pb/²³⁸U age vs. Th/U ratio using available age data obtained from garnet‑biotite gneiss (99122803B) in the Highland Complex.

Inherited domain

Metamorphic and reset domain Discordant data

0.0 0.1 0.2 0.3 0.4

0 2 4 6 8

200

1000

1400

1800

99122803B (HC) 2200

Grt-Bt gneiss

207

Pb/

²³⁵

U

data-point error ellipses are 2

Inherited domains (green) 1980-1500 Ma

Metamorphic and resetdomains (gray)

640-550 Ma 600

540 580

620

206238

P b / U

660

10

1

0.10

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h / U

206

Pb/

²³⁸

U age (Ma)

concordant data accompanying 60 discordant data (Fig. 6.8.1). The internal textures of their cores and rims can be described as bright-CL oscillatory zoned or dark-CL weakly zoned cores and, dark-CL blurry zoned to unzoned rims from CL images, respectively (Fig. 6.8.1). The cores showed the ages ranging from 1860 Ma to 1450 Ma and broad Th/U ratios of 0.08–1.15 with a datum of 600 Ma age and Th/U ratio of 0.01 (Figs.

6.8.1, 6.8.2). The ages and Th/U ratios of dark-CL rims were 620–530 Ma and 0.02–

0.09, respectively (Figs. 6.8.1, 6.8.2).

Garnet gneiss (14111101)

Sample 14111101 contains the zircons with 0.03–0.18 mm in long and rounded, stubby, elongate and dominantly ovoid shape. They provide 48 concordant data from 16 cores and 32 rims and 43 discordant data. The cores are slightly zoned with dark- to brightish-CL and overgrown by dark-CL weakly patchy zoned or unzoned rims (Fig.

6.9.1). The inherited cores exhibited 2080–1630 Ma ages and Th/U ratios of 0.22–0.66, while the rims and the domains with same textures with the rims did 630–520 Ma ages and Th/U ratios of 0.01–0.19 (Figs. 6.9.1, 6.9.2).

Garnet gneiss (14110901A)

The zircons in another sample 14110901A mainly show rounded, elongated shape and 0.04–0.24 mm long. The less amounts of grains which are ovoid with polygonal face and elongate with prismatic face are also observed. 77 concordant data was obtained from 39 cores and 38 rims with 13 discordant data. The internal textures of the cores are weakly oscillatory, sector, patchy zoned or unzoned with dark- to bright-CL (Fig. 6.10.1). The cores are associated with the overgrown rims which are dark- to bright-CL faint sector zoned to unzoned with minor dark-CL weakly oscillatory zoned (Fig. 6.10.1). ²⁰⁶Pb/²³⁸U ages of the cores and weakly oscillatory zoned rims are undistinguishable (1130–710 Ma) with Th/U ratios ranging between 0.02 and 5.20, while those of most rims are 620–520 Ma with 0.06–0.15 Th/U ratios (Figs. 6.10.1, 6.10.2).

Garnet gneiss (99122510A)

Fig. 6.8.1. Cathodoluminescence images of analyzed zircon grains of garnet‑biotite gneiss (99122201B) in the Highland Complex. Analyzed spot,²⁰⁶Pb/²³⁸U age and Th/U ratio are also shown.

Fig. 6.8.2. Concordia diagram and plots of²⁰⁶Pb/²³⁸U age vs. Th/U ratio using available age data obtained from garnet‑biotite gneiss (99122201B) in the Highland Complex.

Inherited domain

Metamorphic and reset domain Discordant data

10

1

0.10

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h/ U

206

Pb/

²³⁸

U age (Ma)

0.0 0.1 0.2 0.3 0.4 0.5

0 2 4 6 8 10

1000

1400

1800

2200

99122201B (HC) Grt-Bt gneiss

207

Pb/

²³⁵

U

206

P b /

238

U

data-point error ellipses are 2

Inherited domains (green) 1860-1450 Ma

540

580

620

Metamorphic and resetdomains (gray)

620-530 Ma 600

Fig. 6.9.1.(Continued).

0.0 0

data-point error ellipses are 2

2 4 6 8 10

0.1 0.2 0.3 0.4 0.5

207

Pb/

²³⁵

U

206

P b /

238

U

14111101 (HC) Grt gneiss

520 560

600 640

680 Inherited domains (green)

2080-1630 Ma

Metamorphic and reset domains (gray) 630-530 Ma

600 1000

1400

1800

2200

Inherited domain

Metamorphic and reset domain Discordant data

10

1

0.1

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h/ U

206

Pb/

²³⁸

U age (Ma)

Fig. 6.10.1. Cathodoluminescence images of analyzed zircon grains of garnet gneiss (14110901A) in the Highland Complex. Analyzed spot,²⁰⁶Pb/²³⁸U age and Th/U ratio are also shown.

Fig. 6.10.1.(Continued).

data-point error ellipses are 2

600

800

1000

1200

0.0 0.1 0.2 0.3 0.4

0 2 4 6 8 10

1000

1400

1800

2200

600

14110901A (HC) Grt gneiss

207

Pb/

²³⁵

U

206

P b /

238

U

Inherited domains (green) 1130-710 Ma

Metamorphic and resetdomains (gray)

620-520 Ma

Inherited domain

Metamorphic and reset domain Discordant data

10

1

0.10

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h / U

206

Pb/

²³⁸

U age (Ma)

The zircon grains with 0.02–0.13 mm long and dominant stubby, some equant, less rounded elongate shapes were observed in sample 99122510A. They provided 69 concordant data (12 cores and 57 rims) and 17 discordant data thought the dating analyses. However, their CL images have a complicacy. The cores represent internal textures of clear concentrically zoning (CL 47) and nebulously zoning with dark- or bright-CL and oscillatory, sector or patchy patterns (Fig. 6.11.1). The rims truncating core textures exhibited dark-CL slightly oscillatory zoned to unzoned, or bright- to brightish-CL weakly sector to patchy zoned texture (Fig. 6.11.1). A datum of concentrically zoned core with CL No. 47 output a 1840 Ma age and Th/U ratio of 0.45, although other cores showed distinct ages and Th/U ratios of 620–490 Ma and 0.02–

0.09, respectively (Figs. 6.11.1, 6.11.2). The overgrown rims gave similar results to younger cores such as 630–500 Ma ages and Th/U ratios of 0.01–0.12 (Figs. 6.11.1, 6.11.2).

Spinel bearing garnet–cordierite–biotite gneiss (94071904A)

In sample 94071904A, zircon grains have various shapes such as rounded equant to squat, ovoid, elongate shapes, and length of 0.02–0.16 mm. Some of ovoid or elongate grains are associated with polygonal or prismatic faces, respectively. The dated zircons provided 49 concordant data and 52 discordant data. The concordant data were obtained from 48 cores and 1 rim. The cores have the variations of internal textures, which are dark- to bright-CL oscillatory, sector zoned or weakly zoned to unzoned textures (Fig.

6.12.1). Most of the cores are accompanied with overgrown rims truncating internal textures of the cores, while some cores are not (Fig. 6.12.1). The overgrown rims with dark-CL weakly zoning exhibited ²⁰⁶Pb/²³⁸U age of 515 Ma and low Th/U ratio (0.03) (Fig. 6.12.1). The result is similar to ²⁰⁶Pb/²³⁸U ages (600–520 Ma) and low Th/U ratios (0.01–0.15) provided from the cores without rims (Figs. 6.12.1, 6.12.2), suggesting the cores formed or recrystallized at the same time of formations of overgrown rims during high-grade metamorphism. On the other hand, the cores with overgrown rims showed older ²⁰⁶Pb/²³⁸U ages of 3420, 2940–1820 Ma and higher Th/U ratios of 0.19–2.06 except 0.04 (Figs. 6.12.1, 6.12.2).

Spinel bearing garnet–sillimanite–cordierite gneiss (99122704G)

Fig. 6.11.1.(Continued).

Fig. 6.11.2. Concordia diagram and plots of²⁰⁶Pb/²³⁸U age vs. Th/U ratio using available age data obtained from garnet gneiss (99122510A) in the Highland Complex.

Inherited domain

Metamorphic and reset domain Discordant data

0.0 0.1 0.2 0.3 0.4

0 2 4 6

1000

1400

1800

99122510A (HC) Grt gneiss

207

Pb/

²³⁵

U

206

P b /

238

U

data-point error ellipses are 2

Inherited domains (green) 1840 Ma

500

540

580

620

660

600

Metamorphic and resetdomains (gray)

630-490 Ma

10

1

0.10

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h / U

206

Pb/

²³⁸

U age (Ma)

Fig. 6.12.1.(Continued).

Inherited domain

Metamorphic and reset domain Discordant data

10

1

0.1

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h/ U

206

Pb/

²³⁸

U age (Ma)

data-point error ellipses are 2

0.0 0.2 0.4 0.6 0.8

0 10 20 30 40

207

Pb/

²³⁵

U

206

P b /

238

U

1400 1800

2200

2600

3000

3400

94071904A (HC) Spl bg. Grt-Crd-Bt gneiss

500

540

580

620

Metamorphic and reset domains (gray)

600-520 Ma

Inherited domains (green) 3440, 2940-1820 Ma

Sample 99122704G contains zircon grains which are 0.04–0.21 mm in long, and dominantly stubby and ovoid with polygonal face, minorly rounded elongate in shape.

59 concordant (45 cores and 14 rims) and 23 discordant data were given by analyzed zircon grains. Their cores showed a few CL patterns of bright-CL oscillatory and sector zoning, dark-CL clear to unclear sector zoning or non-zoning (Fig. 6.13.1). In contrast, the rims have simple CL pattern of darkish- to bright-CL sector zoning (Fig. 6.13.1).

The cores with bright-CL exhibited 2040–1690 Ma ages with Th/U ratios from 0.18 to 0.84, and 550–510 Ma ages with Th/U ratios of 0.01–0.02, 0.15 (Figs. 6.13.1, 6.13.2).

On the other hand, those with dark-CL did 620–500 Ma ages and Th/U ratios from 0.01 to 0.07 with one analysis indicative of 1810 Ma age and Th/U ratio of 0.01 (Figs. 6.13.1, 6.13.2). The ages and Th/U ratios obtained from the rims were 610–470 Ma and 0.01–

0.12, respectively (Figs. 6.13.1, 6.13.2).

Garnet–orthopyroxene–biotite gneiss (99122101D1)

Zircons in this rock type have dominant rounded stubby and ovoid shapes, less elongate shape and stubby shape with polygonal faces, and the length of 0.02–0.12 mm.

The zircons consist of inherited cores and overgrown rims under the CL images. The cores are sector, patchy zoned or faint to unzoned with dark- to bright-CL (Fig. 6.14.1).

The internal textures of the rims are dark-CL weakly zoning or bright-CL sector zoning (Fig. 6.14.1). 83 analysis points exhibited 57 concordant data derived from 52 cores and 5 rims, and 30 discordant data. ²⁰⁶Pb/²³⁸U ages and Th/U ratios of the cores were 1010–

670, 610–500 Ma and 0.02–0.11, respectively, except for one analysis indicative of 2560 Ma age and Th/U ratio of 0.86 (Figs. 6.14.1, 6.14.2). The overgrown rims yielded

206Pb/²³⁸U ages of 590–510 Ma and Th/U ratios of 0.01–0.11, which are quite similar to those of younger cores (Figs. 6.14.1, 6.14.2).

Garnet–orthopyroxene–biotite granulite (99122101D2)

The morphology of zircons in this rock is 0.03–0.18 mm long, dominant stubby with polygonal face and ovoid with minor elongate shapes. 44 concordant data were obtained from 37 cores and 7 rims with 5 discordant data. Under the CL images, the cores are bright-CL oscillatory zoned, dark-CL banding zoned and dark- to bright-CL clear to weakly sector zoned, while the rims are dark- to brightish-CL sector zoned to

Fig. 6.13.1.(Continued).

Inherited domain

Metamorphic and reset domain Discordant data

450

650

550

750

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0 4 8 12 16 20

1400

1800

2200

2600

99122704G 3000

Spl bg. Grt-Sil-Crd gneiss

207

Pb/

²³⁵

U

206

P b /

238

U

data-point error ellipses are 2

Inherited domains (green) 2040-1600 Ma

Metamorphic and resetdomains (gray)

620-470 Ma

10

1

0.10

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h / U

206

Pb/

²³⁸

U age (Ma)

Fig. 6.14.1. Cathodoluminescence images of analyzed zircon grains of garnet‑orthopyroxene‑biotite gneiss (99122101D1) in the Highland Complex. Analyzed spot,²⁰⁶Pb/²³⁸U age and Th/U ratio are also shown.

Fig. 6.14.2. Concordia diagram and plots of²⁰⁶Pb/²³⁸U age vs. Th/U ratio using available age data obtained from garnet‑orthopyroxene‑biotite gneiss (99122101D1) in the Highland Complex.

Inherited domain

Metamorphic and reset domain Discordant data

data-point error ellipses are 2

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0 2 4 6 8 10 12 14

207

Pb/

²³⁵

U

206

P b /

238

U

99122101D1 (HC) Grt-Opx-Bt gneiss

1000 1400

1800

2200

2600

600

Metamorphic and reset domains (gray)

610-500 Ma

700

900 800

600

1000 Inherited domains (green)

2560, 1010-670 Ma

10

1

0.1

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h/ U

206

Pb/

²³⁸

U age (Ma)

unzoned (Fig. 6.15.1). The oscillatory and banding zoned cores showed ²⁰⁶Pb/²³⁸U ages of 2520, 2210 and 1720 Ma and high Th/U ratios of 0.25–0.81, sector zoned cores did

206Pb/²³⁸U ages of 870–710, 640–470 Ma and low Th/U ratios of 0.01–0.17 (Figs. 6.15.1, 6.15.2). The rims truncating cores gave the ages of 530–470 Ma with low Th/U ratios of 0.02–0.15 (Figs. 6.15.1, 6.15.2).

Garnet–orthopyroxene–biotite gneiss (13040106C)

In sample 13040106C, zircons have the length of 0.03–0.36 mm and rounded stubby, ovoid and dominant elongate shapes. 36 concordant data and 48 discordant data were detected from the zircon grains whose internal textures are composed of cores and rims. The cores, which are truncated by bright-CL sector zoned rims, are clear to blurry oscillatory, sector zoned or unzoned with dark- to bright-CL (Fig. 6.16.1). According to internal textures of cores, the concordant data showed different ages and Th/U ratios.

The oscillatory zoned cores showed older ²⁰⁶Pb/²³⁸U ages of 1820–1530 Ma with higher Th/U ratios of 0.44–1.62, while dark-CL weakly zoned to unzoned cores did younger ages of 630–540 Ma with lower Th/U ratios of 0.02–0.08 except 0.53 (Figs. 6.16.1, 6.16.2). ²⁰⁶Pb/²³⁸U ages of 560–490 Ma and Th/U ratios of 2.56–3.98 were obtained from the bright-CL sector zoned domains of cores and rims (Figs. 6.16.1, 6.16.2).

Siliceous garnet–sillimanite gneiss (13040102A)

Sample 13040102A contains zircon grains which have rounded stubby and ovoid with minor equant and elongate shapes and 0.03–0.16 mm long. The zircons provided 22 concordant data from 17 cores and 5 rims with 29 discordant data. The inherited cores kept the various internal textures which are darkish- to bright-CL oscillatory, sector, patchy zoned or dark-CL faint to unzoned (Fig. 6.17.1). The dark-CL faint to unzoned rims overgrew around the inherited cores (Fig. 6.17.1). ²⁰⁶Pb/²³⁸U ages obtained from the cores varied between 3050 Ma and 1870 Ma with Th/U ratios of 0.22–3.80, while those from the rims were 640–580 Ma with Th/U ratios of 0.09–0.27 (Figs. 6.17.1, 6.17.2).

6.1.2.3. Metaigneous rocks from the Wanni Complex

Fig. 6.15.1. Cathodoluminescence images of analyzed zircon grains of garnet‑orthopyroxene‑biotite granulite (99122101D2) in the Highland Complex. Analyzed spot,²⁰⁶Pb/²³⁸U age and Th/U ratio are also shown.

Fig. 6.15.2. Concordia diagram and plots of ²⁰⁶Pb/²³⁸U age vs. Th/U ratio using available age data obtained from garnet‑orthopyroxene‑biotite granulite (99122101D2) in the Highland Complex.

Inherited domain

Metamorphic and reset domain Discordant data

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0 2 4 6 8 10 12 14

1000

1400

1800

2200

2600

99122101D2 (HC) Grt-Opx-Bt granulite

207

Pb/

²³⁵

U

206

P b /

238

U

data-point error ellipses are 2

Inherited domains (green) 2500, 2210, 1720, 870-710 Ma

500 600

700 800

900

600

Metamorphic and resetdomains (gray)

640-470 Ma

10

1

0.10

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h / U

206

Pb/

²³⁸

U age (Ma)

Fig. 6.16.1.(Continued).

Inherited domain

Metamorphic and reset domain Discordant data

0.0 0.1 0.2 0.3 0.4

0 2 4 6

440 480

520 560

600 640

680 13040106C (HC)

Grt-Opx-Bt gneiss

data-point error ellipses are 2

Inherited domains (green) 1820-1530 Ma

206

P b /

238

U

207

Pb/

²³⁵

U

600

1000

1400

1800

Metamorphic and reset domains (gray) 630-490 Ma

10

1

0.10

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h/ U

206

Pb/

²³⁸

U age (Ma)

Fig. 6.17.1. Cathodoluminescence images of analyzed zircon grains of siliceous garnet‑sillimanite gneiss (13040102A) in the HIghland Complex. Analyzed spot,²⁰⁶Pb/²³⁸U age and Th/U ratio are also shown.

Inherited domain

Metamorphic and reset domain Discordant data

data-point error ellipses are 2

0.0 0.2 0.4 0.6 0.8

0 4 8 12 16 20 24

206

P b /

238

U

207

Pb/

²³⁵

U

1000 1400

1800

2200

2600

3000

13040102A (HC) Siliceous Grt-Sil gneiss

Inherited domains (green) 3050, 2700-1870 Ma

550 570

590 610

630 650

670

Metamorphic and reset domains (gray) 640-580 Ma

10

1

0.10

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h /U

206

Pb/

²³⁸

U age (Ma)

Garnet–two-pyroxene granulite (14111401)

Dominant stubby and ovoid zircons with polygonal faces and minor rounded elongate ones are observed in sample 14111401. Zircons, which are 0.02–0.1 mm in long, provided 31 concordant data (9 cores and 22 rims) and 4 discordant data. The cores showed two types of internal textures, which are bright-CL sector or patchy zoned and dark-CL patchy zoned textures (Fig. 6.18.1). These cores are overgrown by dark-CL sector or faint zoned rims (Fig. 6.18.1). Bright-CL cores showed the oldest age of 883 Ma with high Th/U ratio of 1.38 and 560–500 Ma ages with Th/U ratios of 0.07–

0.67, while dark-CL cores did ²⁰⁶Pb/²³⁸U ages of 700–600 Ma and Th/U ratios ranging between 0.29 and 1.51 (Figs. 6.18.1, 6.18.2). ²⁰⁶Pb/²³⁸U ages and Th/U ratios, which were detected from dark-CL rims, varied from 630 Ma to 500 Ma and from 0.05 to 0.85, respectively (Figs. 6.18.1, 6.18.2).

Two-pyroxene granulite (14031705E-1)

Zircon grains in sample 14031705E-1 are 0.03–0.17 mm long and dominantly stubby, ovoid, and less elongated in shape. Many of ovoid and elongated grains are associated with polygonal faces. These zircons provided 50 concordant analyses (27 cores and 23 rims) and 13 discordant analyses. Cores can be divided into inner core and outer core which showed the internal textures of dark-CL faint zoning and bright-CL sector zoning or non-structure, respectively (Fig. 6.19.1). The cores are rimmed by dark-CL unzoned domains (Fig. 6.19.1). Concordant core analyses yield scattered

206Pb/²³⁸U ages of 620–500 Ma and Th/U ratios of 0.15–0.42, whereas the concordant rims have ages of 620–520 Ma with Th/U ratios of 0.11–0.29 (Figs. 6.19.1, 6.19.2). In addition, 8 discordant analyses defined a discordia intercepted at 1935 ± 100 Ma and 597 ± 51 Ma (MSWD = 2.3) (Fig. 6.19.2).

Two-pyroxene granulite (14111204B)

Zircons in sample 14111204B are rounded stubby, ovoid and elongate in shape and 0.03–0.16 mm in long. Minor ovoid zircons have polygonal faces. The zircons showed 28 concordant data from 7 cores and 21 rims, and 11 discordant data. The cores have complicated zonation like a mixture of bright-CL sector zoned domain and dark-CL

Fig. 6.18.1.(Continued).

0.05 0.07 0.09 0.11 0.13 0.15 0.17

0.0 0.4 0.8 1.2 1.6 2.0 2.4

Inherited domains? (red) 880 Ma

14111401 (WC) Grt-two-Px granulite

data-point error ellipses are 2

Metamorphic and resetdomains (gray) 630-500 Ma

206

P b /

238

U

207

Pb/

²³⁵

U

400 500

600 700

800

900

1000

10

1

0.1

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h/ U

206

Pb/

²³⁸

U age (Ma)

Inherited domain

Metamorphic and reset domain Discordant data

Omitted data

Fig. 6.19.1. Cathodoluminescence images of analyzed zircon grains of two‑pyroxene granulite (14031705E‑1) in the Wanni Complex. Analyzed spot,²⁰⁶Pb/²³⁸U age and Th/U ratio are also shown.

Fig. 6.19.2. Concordia diagram and plots of ²⁰⁶Pb/²³⁸U age vs. Th/U ratio using available age data obtained from two‑pyroxene granulite (14031705E‑1) in the Wanni Complex.

10

1

0.1

0.01

0.001

500 1500

0 1000 2000 2500 3000 3500

T h / U

206

Pb/

²³⁸

U age (Ma)

Metamorphic and reset domain Discordant data

0.0 0.1 0.2 0.3

0 1 2 3 4 5 6

14031705E-1 (WC) Two-Px granulite

data-point error ellipses are 2

206

P b /

238

U

207

Pb/

²³⁵

U

500 540

580 620

660

Discordia (white) (n=8) Upper intercept 1935 ± 100 Ma

Lower intercept 597 ± 51 Ma MSWD = 2.3

600

1000

1400

1800

Metamorphic and resetdomains (gray) 620-500 Ma

patchy zoned domain (Fig. 6.20.1). They accompany with dark- to brightish-CL sector or weakly zoned rims (Fig. 6.20.1). ²⁰⁶Pb/²³⁸U ages of the cores scattered between 860 Ma and 680 Ma, while those of the rims ranged from 620 Ma to 490 Ma (Figs. 6.20.1, 6.20.2). Th/U ratios of cores and rims are not so different and varied from 0.25 to 1.02.

Two-pyroxene granulite (14111404C)

The zircon grains have the length of 0.04–0.21 mm, and dominant rounded elongate and rare stubby shapes. 35 concordant data was detected from 20 cores and 15 rims of zircon grains with 15 discordant data. The CL images of cores represented dark- to bright-CL sector or patchy zoning, and brightish-CL weakly oscillatory zoning, whist those of rims did dark- to brightish-CL weakly sector zoning and dark-CL homogenous texture (Fig. 6.21.1). The concordant data from cores showed the scattered ages of 1040–750 Ma and minor 660–530 Ma with Th/U ratios between 0.02 and 1.24 (Figs.

6.21.1, 6.21.2). Similarly the data from rims did the ages of 600–510 Ma and minor 940–770 Ma with Th/U ratios of 0.19–0.98 (Figs. 6.21.1, 6.21.2).

Two-pyroxene granulite (14031702B)

Sample 14031702B contains zircon grains which show rounded stubby, ovoid, elongate shape and 0.03–0.17 mm long. The polygonal faces are associated with minor ovoid zircons. The zircons provided 30 concordant data (24 cores and 6 rims) and 17 discordant data. Under the CL images, the cores indicated darkish- to bright-CL oscillatory, sector zoned or dark-CL blurry to unzoned textures, while the rims did dark-CL weakly zoned textures (Fig. 6.22.1). The analyses of cores and rims provided

206Pb/²³⁸U ages of 990–850 Ma with Th/U ratios of 0.06–1.36 and 590–510 Ma with Th/U ratios of 0.01–1.01, respectively (Figs. 6.22.1, 6.22.2). Although the cores have scattered ages and Th/U ratios, there are no relations with internal textures. However, the ages obtained from the cores clustered at ca. 990 Ma which is the oldest concordant datum in this sample (Fig. 6.22.2).

Two-pyroxene gneiss (14111301A)

The lengths of zircon grains in this sample are varied between 0.02 and 0.17 mm.

Some of them are ovoid with rounded or polygonal face, but others are elongate with

ドキュメント内 Petrology of high-temperature metamorphic rocks in the Highland and Wanni Complexes, Sri Lanka (ページ 109-200)