As described in the Outline of Phonology (OP2) from the 1950s, the combinations rg, lg became rr, ll by time of Tarquesta [TQ], probably passing through lʒ, rʒ [lɣ, rɣ] in the process. However, this sound change only occurred in the Ñoldorin dialect of Quenya:

The products of rg, lg; rgw, lgw are in (N.) TQ rr, ll (but rgē̆, lgē̆ > rye, lye); rw, lw; in Vanyarin rg, lg, rw, lw. The TQ (Noldorin) forms, and especially the fact that even in PQ the product of rgw, lgw is rw, lw, point to the opening of g > ʒ ... The Vanyarin rg, lg must then be produced by re-stopping (OP2: PE19/93-94).

Tolkien described similar changes in the Outline of Phonetic Development (OP1) from the 1940s, but in this earlier document the sound change was limited to the Lindarin [pre-Vanyarin] dialect:

After l, r. In PQ the stops remained: ld, rd, lg, rg, lb, rb. lgw, rgw became lw, rw already in early PQ ... In Lindarin ld, rd, lb still remained; but rb > rv; while lg, rg > lʒ, rʒ > ll, rr, but rgē̆, lgē̆ > rye, lye (OP1: PE19/46).

Thus in the 1930s, voiced stops did not become spirants after liquids, except only in the limited case of lgw, rgw > lw, rw and in Lindarin TQ for a few combinations, of which lg, rg > lʒ, rʒ > ll, rr. However, in the 1950s, voiced stops did become spirants after liquids, only to be restopped in the Vanyarin dialect but in the Ñoldorin dialect developing onwards, so that lʒ, rʒ > ll, rr. The main exceptions in both conceptual periods are rgw, lgw > rw, lw and rge, lge > rye, lye, sound changes that occurred in PQ or earlier (in the 1950s, at least).

There are very few late examples of lg, rg > lʒ, rʒ > ll, rr. The most obvious example I’ve found is in the Quendi and Eldar essay from around 1960:

• Valarin delgūmā > Q. telluma “dome” (WJ/399); presumably *telguma in the Vanyarin Quenya dialect.

There is an example in The Etymologies of the 1930s with a somewhat different phonetic development:

• ᴹ✶ulgundō > ᴹQ. ulundo “monster” (Ety/ÚLUG).

Here it seems lgu > lʒu > lu, perhaps a variation on lgw > lw, but that was not the development seen in later Q. telluma.

This particular sound change (lʒ, rʒ > Ñoldorin TQ ll, rr but Vanyarin TQ lg, rg) presents an interesting problem in establishing the timeline of the divergence of the Ñoldorin and Vanyarin dialects. In both these dialects, normally ʒ from g vanished, and it would make sense if the changes to lʒ, rʒ occurred before the general vanishing of ʒ. However, it is known that ʒ vanished in Early Parmaquesta [PQ] (PE19/70).

I see two possible explanations. First, that ʒ lingered longer in the combinations lʒ, rʒ, and then developed further only in the TQ period with divergent changes in the two dialects. Second, that the restopping of voiced spirants after liquids was a PQ divergence of the Ñoldorin and Vanyarin dialects (the latter perhaps influenced by the Valarin language), but the two populations remained in sufficient contact that their dialects shared other sound changes in the PQ period. Of the two theories, I find the first more compelling, and it probably means that the combinations lƀ, rƀ [lβ, rβ] also lingered for a while, diverging to Ñoldorin lv, rv and Vanyarin lb, rb only in the TQ period. This might help explain the occasional appearance of lb in Exilic Ñoldorin Quenya.

Conceptual Development: In draft notes for the Qenya Phonology of the 1910s Tolkien said “lɣ, rɣ ... gave lg, rg” (PE12/24). However, there are no words with the combinations lg, rg in the Qenya Lexicon from the 1910s, and there are a few examples from Noldorin word lists in the 1920s that show different phonetic developments:

• ᴱ✶balga > ᴱQ. palla “paunch” (PE13/138).
• ᴱ✶mḷgo > ᴱQ. millo “oil” (PE13/139).
• ᴱ✶grgu- > ᴱQ. karko “alimentary canel” (PE13/144).

Thus it seems lg > ll already in the 1920s, but rg > rk.

@@@ similarly [lg]?